dpans/concept-eval.tex

% -*- Mode: TeX -*-
%%Evaluation

\term{Execution} of \term{code} can be accomplished by a variety of means ranging
from direct interpretation of a \term{form} representing a \term{program}
to invocation of \term{compiled code} produced by a \term{compiler}.
 
\newtermidx{Evaluation}{evaluation} is the process by which a \term{program} is \term{executed} in \clisp.
The mechanism of \term{evaluation} is manifested
 both implicitly through the effect of the \term{Lisp read-eval-print loop},
 and  explicitly through the presence of the \term{functions} 
       \funref{eval},
       \funref{compile},
       \funref{compile-file},
   and \funref{load}.
Any of these facilities might share the same execution strategy, 
or each might use a different one.

The behavior of a \term{conforming program} processed by \funref{eval}
and by \funref{compile-file} might differ; \seesection\SemanticConstraints.

\term{Evaluation} can be understood in terms of a model in which an
interpreter recursively traverses a \term{form} performing each
step of the computation as it goes.  
This model, which describes the semantics of \clisp\ \term{programs},
is described in \secref\EvaluationModel.

\beginsubSection{Introduction to Environments}
\DefineSection{IntroToEnvs}

A \newterm{binding} is an association between a \term{name} and
that which the name denotes.  \term{Bindings} are \term{established}
in a \term{lexical environment} or a \term{dynamic environment}
by particular \term{special operators}.

An \newterm{environment} is a set of \term{bindings} and other information
used during evaluation (\eg to associate meanings with names).

\term{Bindings} in an \term{environment} are partitioned into \newtermidx{namespaces}{namespace}.
A single \term{name} can simultaneously have more than one
associated \term{binding} per \term{environment},
but can have only one associated \term{binding} per \term{namespace}.

%!!!! Moon: There out to be a list of the namespaces!!
%           Do they even have agreed-upon names?

\beginsubsubsection{The Global Environment}

The \newterm{global environment} is that part of an \term{environment}
that contains \term{bindings} with both \term{indefinite scope} 
and \term{indefinite extent}.
The \term{global environment} contains, among other things, the following:

\beginlist
\item{\bull} \term{bindings} of \term{dynamic variables} and \term{constant variables}.
\item{\bull} \term{bindings} of \term{functions}, \term{macros}, and \term{special operators}.
\item{\bull}%
\issue{DEFINE-COMPILER-MACRO:X3J13-NOV89}%
 \term{bindings} of \term{compiler macros}.
\endissue{DEFINE-COMPILER-MACRO:X3J13-NOV89}%
\item{\bull} \term{bindings} of \term{type} and \term{class} \term{names}
\item{\bull} information about \term{proclamations}.
\endlist

\endsubsubsection%{The Global Environment}

\beginsubsubsection{Dynamic Environments}

A \newterm{dynamic environment} for \term{evaluation} is that part of an
\term{environment} that contains \term{bindings} whose duration
is bounded by points of \term{establishment} and \term{disestablishment} 
within the execution of the \term{form} that
established the \term{binding}.
A \term{dynamic environment} contains, among other things, the following:

\beginlist
\item{\bull} \term{bindings} for \term{dynamic variables}.
\item{\bull} information about \term{active} \term{catch tags}.
\item{\bull} information about \term{exit points} established by \specref{unwind-protect}.
\item{\bull} information about \term{active} \term{handlers} and \term{restarts}.
\endlist

The \term{dynamic environment} that is active at any given point 
in the \term{execution} of a \term{program} is referred to by 
definite reference as ``the current \term{dynamic environment},''
or sometimes as just ``the \term{dynamic environment}.''

Within a given \term{namespace},
a \term{name} is said to be \term{bound}
in a \term{dynamic environment} if there is a \term{binding} 
%or mapping
associated with its \term{name} in the \term{dynamic environment} 
or, if not, there is a \term{binding} 
%or mapping
associated with its name in the \term{global environment}.

\endsubsubsection%{Dynamic Environments}

\beginsubsubsection{Lexical Environments}

A \newterm{lexical environment} for \term{evaluation} at some position in a \term{program}
is that part of the \term{environment} that contains information having 
\term{lexical scope} within the \term{forms} containing that position.
A \term{lexical environment} contains, among other things, the following:

\beginlist
\item{\bull} \term{bindings} of \term{lexical variables} and \term{symbol macros}.
\item{\bull} \term{bindings} of \term{functions} and \term{macros}.
             (Implicit in this is information about those \term{compiler macros} 
	      that are locally disabled.)
%% Replaced by parenthetical remark on previous bullet. -kmp 5-Dec-91
% \item{\bull} \term{bindings} of \term{compiler macros}
% 	     (specifically, information about those \term{compiler macros} 
% 	      that are locally disabled)
\item{\bull} \term{bindings} of \term{block tags}.
\item{\bull} \term{bindings} of \term{go tags}.
\item{\bull} information about \term{declarations}.
\endlist

The \term{lexical environment} that is active at any given position
in a \term{program} being semantically processed is referred to by
definite reference as ``the current \term{lexical environment},''
or sometimes as just ``the \term{lexical environment}.''  

Within a given \term{namespace},
a \term{name} is said to be \term{bound} in a \term{lexical environment}
if there is a \term{binding} 
%or mapping
associated with its \term{name}
in the \term{lexical environment} or, if not, there is a \term{binding} 
%or mapping
associated with its name in the \term{global environment}.

\beginsubsubsubsection{The Null Lexical Environment}
\DefineSection{NullLexicalEnv}

The \newterm{null lexical environment} is equivalent to the \term{global environment}.

Although in general the representation of an \term{environment} \term{object}
is \term{implementation-dependent}, \nil\ can be used in any situation where an
\term{environment} \term{object} is called for in order to denote 
the \term{null lexical environment}.

\endsubsubsubsection%{The Null Lexical Environment}

\endsubsubsection%{Lexical Environments}

\beginsubsubsection{Environment Objects}
\DefineSection{EnvObjs}

Some \term{operators} make use of an \term{object}, 
called an \newterm{environment object},
that represents the set of \term{lexical bindings} needed to perform
semantic analysis on a \term{form} in a given \term{lexical environment}.
The set of \term{bindings} in an \term{environment object}
may be a subset of the \term{bindings} that would be needed to actually 
perform an \term{evaluation}; for example, \term{values} associated with
\term{variable} \term{names} and \term{function names} in the corresponding
\term{lexical environment} might not be available in an \term{environment object}.

The \term{type} and nature of an \term{environment object} is \term{implementation-dependent}.
The \term{values} of \term{environment parameters} to \term{macro functions}
are examples of \term{environment objects}.

The \term{object} \nil\ when used as an \term{environment object}
denotes the \term{null lexical environment};
\seesection\NullLexicalEnv.

\endsubsubsection%{Environment Objects}

\endSubsection%{Introduction to Environments}

\beginSubsection{The Evaluation Model}
\DefineSection{EvaluationModel}

A \clisp\ system evaluates \term{forms} with respect to lexical,
dynamic, and global \term{environments}.  The following sections
describe the components of the \clisp\ evaluation model.

\beginsubsubsection{Form Evaluation}

%% 5.1.0 2
%% 5.1.1 1
\term{Forms} fall into three categories:
%!!!Is this a correct use of self-evaluating? -kmp 09-Apr-91
\term{symbols}, \term{conses}, and \term{self-evaluating objects}.
The following sections explain these categories.
                                            
\beginsubsubsubsection{Symbols as Forms}
\DefineSection{SymbolsAsForms}

If a \term{form} is a \term{symbol},
then it is either a \term{symbol macro} or a \term{variable}.

The \term{symbol} names a \term{symbol macro} 
if there is a \term{binding} of the \term{symbol} as a \term{symbol macro}
in the current \term{lexical environment} 
\issue{ISO-COMPATIBILITY:ADD-SUBSTRATE}
 (see \macref{define-symbol-macro} and \specref{symbol-macrolet}).
\endissue{ISO-COMPATIBILITY:ADD-SUBSTRATE}
If the \term{symbol} is a \term{symbol macro},
its expansion function is obtained.
The expansion function is a function of two arguments, and is invoked
by calling the \term{macroexpand hook} with 
     the expansion function as its first argument,
     the \term{symbol} as its second argument,
 and an \term{environment object} (corresponding to the current \term{lexical environment})
      as its third argument.
The \term{macroexpand hook}, in turn, calls the expansion function with the
\term{form} as its first argument and the \term{environment} as its second argument.
The \term{value} of the expansion function, which is passed through
by the \term{macroexpand hook}, is a \term{form}. 
This resulting \term{form} is processed in place of the original \term{symbol}.

If a \term{form} is a \term{symbol} that is not a \term{symbol macro},
then it is the \term{name} of a \term{variable}, and the \term{value} of that
\term{variable} is returned. There are three kinds of variables:
 \term{lexical variables},
 \term{dynamic variables},
and
 \term{constant variables}.
% global variables.
A \term{variable} can store one \term{object}.
The main operations on a \term{variable} are 
 to \term{read}\meaning{1} and 
 to \term{write}\meaning{1}
its \term{value}.

%% 7.1.1 1
%% 7.2.0 2
%% 7.2.0 3

An error \oftype{unbound-variable} should be signaled if
an \term{unbound variable} is referenced.

\term{Non-constant variables} can be \term{assigned} by using \specref{setq} 
or \term{bound}\meaning{3} by using \specref{let}.
\Thenextfigure\ lists some \term{defined names} that
are applicable to assigning, binding, and defining \term{variables}.

\displaythree{Some Defined Names Applicable to Variables}{
boundp&let&progv\cr
defconstant&let*&psetq\cr
defparameter&makunbound&set\cr
defvar&multiple-value-bind&setq\cr
lambda&multiple-value-setq&symbol-value\cr
}

The following is a description of each kind of variable.

\beginsubsubsubsubsection{Lexical Variables}

%!!! Issue of (let ((x x)) (declare (special x)) ...). Is the outer X special? -kmp
%% 5.1.2 2
%% Rewritten by KAB and KMP.
% A \term{variable} that occurs textually within a \term{form}
% that creates a \term{lexical binding} for a \term{variable}
% of the same name is a \term{lexical variable} unless the reference is locally
% declared \declref{special} (see \misc{declare}) or the \term{binding}
% is shadowed by a \term{form} that creates a \term{dynamic binding} of the same name.

%% 5.1.2 3
A \term{lexical variable} is a \term{variable} that can be referenced only within 
the \term{lexical scope} of the \term{form} that establishes that \term{variable};
\term{lexical variables} have \term{lexical scope}.
Each time a \term{form} creates a \term{lexical binding} of a \term{variable},
a \term{fresh} \term{binding} is \term{established}.

Within the \term{scope} of a \term{binding} for a \term{lexical variable} \term{name},
uses of that \term{name} as a \term{variable} are considered to be references
to that \term{binding} except where the \term{variable} is \term{shadowed}\meaning{2} 
by a \term{form} that \term{establishes} a \term{fresh} \term{binding} for that 
\term{variable} \term{name},
or by a \term{form} that locally \term{declares} the \term{name} \declref{special}.

A \term{lexical variable} always has a \term{value}.
There is no \term{operator} that introduces a \term{binding} for a
\term{lexical variable} without giving it an initial \term{value}, nor
is there any \term{operator} that can make a \term{lexical variable} be \term{unbound}.

\term{Bindings} of \term{lexical variables} are found in the \term{lexical environment}.

\endsubsubsubsubsection%{Lexical Variables}

\beginsubsubsubsubsection{Dynamic Variables}

A \term{variable} is a \term{dynamic variable} if one of the following
conditions hold:

\beginlist

 \item{\bull} It is locally declared or globally proclaimed \declref{special}.

 \item{\bull} It occurs textually within a \term{form} that
creates a \term{dynamic binding} for a \term{variable} of the \term{same} \term{name},
and the \term{binding} is not \term{shadowed}\meaning{2} by a \term{form}
that creates a \term{lexical binding} of the same \term{variable} \term{name}.

%% Is this right? Is there an issue that applies to this? -kmp
%% Removed for now because Barrett and Moon balked, too. -kmp 13-Oct-91
%  \item{\bull} It is not locally declared \declref{special}, 
%  and it does not occur textually within a \term{form} that creates
%  a lexical \term{binding} for a variable of the same name.

\endlist

%!!! Barrett wants to mention "dynamic scope" here someplace.
A \term{dynamic variable} can be referenced at any time in any \term{program};
there is no textual limitation on references to \term{dynamic variables}.
At any given time, all \term{dynamic variables} with a given name refer to 
exactly one \term{binding}, either in the \term{dynamic environment}
or in the \term{global environment}.

%% 5.1.2 5
The \term{value} part of the \term{binding} for a \term{dynamic variable} might
be empty; in this case, the \term{dynamic variable} is said to have no \term{value},
or to be \term{unbound}.  A \term{dynamic variable} can be made \term{unbound}
by using \funref{makunbound}.
           
The effect of \term{binding} a \term{dynamic variable} is to create
a new \term{binding} to which all references to that \term{dynamic variable}
in any \term{program} refer for the duration of the \term{evaluation} of the \term{form}
that creates the \term{dynamic binding}.

% %!!! Need to merge this in with previous or delete it.
% %% 5.1.2 4
% A \term{dynamic variable} that is referenced outside the \term{dynamic extent} of
% a \term{form} that \term{binds} the \term{symbol} that names the
% variable is a global variable. A global variable is \term{unbound}
% unless and until explicitly assigned a value, except for global
% variables defined in this specification or by an implementation.

A \term{dynamic variable} can be referenced outside the \term{dynamic extent} of
a \term{form} that \term{binds} it.  Such a \term{variable} is sometimes called 
a ``global variable'' but is still in all respects just a \term{dynamic variable}
whose \term{binding} happens to exist in the \term{global environment} rather than in some
\term{dynamic environment}.

% slight rewording  --sjl 3 Mar 92
A \term{dynamic variable} is \term{unbound}
unless and until explicitly assigned a value, except for 
those variables whose initial value is 
defined in this specification or by an \term{implementation}.

%% Pretty much said above.
% \term{Bindings} of \term{dynamic variables} are found in the \term{dynamic environment}
% or the \term{global environment}.

\endsubsubsubsubsection%{Dynamic Variables}

\beginsubsubsubsubsection{Constant Variables}
\DefineSection{ConstantVars}

%% 1.2.5 6
%% 5.1.2 6
Certain variables, called \term{constant variables}, are reserved as ``named constants.''  
The consequences are undefined if an attempt is made to 
    assign a value to,
 or create
a \term{binding} for a \term{constant variable}, 
except that a `compatible' redefinition of a \term{constant variable}
using \macref{defconstant} is permitted; \seemac{defconstant}.

\term{Keywords}, 
\term{symbols} defined by \clisp\ or the \term{implementation}
  as constant (such as \nil, \t, and \conref{pi}),
and \term{symbols} declared as constant using \macref{defconstant}
are \term{constant variables}.

\endsubsubsubsubsection%{Constant Variables}

\beginsubsubsubsubsection{Symbols Naming Both Lexical and Dynamic Variables}

%A \term{symbol} can name both a \term{lexical variable} and a \term{dynamic variable}.
%%KAB: The above is a little confusing, as it could be read to mean they 
%%     are somehow in different namespaces.
%%KMP: Replaced by the following section:

The same \term{symbol} can name both 
    a \term{lexical variable} 
and a \term{dynamic variable},
but never in the same \term{lexical environment}.

In the following example, the \term{symbol} \f{x} is used,
at different times, 
    as the \term{name} of a \term{lexical variable}
and as the \term{name} of a \term{dynamic variable}.
 
\code
 (let ((x 1))            ;Binds a special variable X
   (declare (special x))
   (let ((x 2))          ;Binds a lexical variable X
     (+ x                ;Reads a lexical variable X
        (locally (declare (special x))
                 x))))   ;Reads a special variable X
\EV 3
\endcode

\endsubsubsubsubsection%{Symbols Naming Both Lexical and Dynamic Variables}

\endsubsubsubsection%{Symbols as Forms}

\beginsubsubsubsection{Conses as Forms}

A \term{cons} that is used as a \term{form} is called a \term{compound form}.

If the \term{car} of that \term{compound form} is a \term{symbol}, 
that \term{symbol} is the \term{name} of an \term{operator},
and the \term{form} is either a \term{special form}, a \term{macro form},
or a \term{function form}, depending on the \term{function} \term{binding} 
of the \term{operator} in the current \term{lexical environment}.
If the \term{operator} is neither a \term{special operator}
nor a \term{macro name}, it is assumed to be a \term{function name}
(even if there is no definition for such a \term{function}).

If the \term{car} of the \term{compound form} is not a \term{symbol},
then that \term{car} must be a \term{lambda expression},
in which case the \term{compound form} is a \term{lambda form}.

How a \term{compound form} is processed depends on whether it is 
classified as a \term{special form}, a \term{macro form}, 
a \term{function form}, or a \term{lambda form}.

\beginsubsubsubsubsection{Special Forms}

% special form changed to special operator in a few places  --sjl 3 Mar 92
A \term{special form} is a \term{form} with special syntax,
special evaluation rules, or both, possibly manipulating the
evaluation environment, control flow, or both.
% When the operator names a \term{special form},
% the operation is performed using special purpose code 
% in the current \term{lexical environment} and \term{dynamic environments}.
%% Simplified for Sandra:
A \term{special operator} has access to
    the current \term{lexical environment} 
and the current \term{dynamic environment}.
Each \term{special operator} defines the manner in which its \term{subexpressions}
are treated---which are \term{forms}, which are special syntax, \etc.

Some \term{special operators} create new 
lexical or dynamic \term{environments} for use during the 
\term{evaluation} of \term{subforms}
of the \term{special form}.  For example, \specref{block} creates a
new \term{lexical environment} that is the same as the one in force
at the point of evaluation of the \specref{block} \term{form}
with the addition of a \term{binding} of the \specref{block} name
to an \term{exit point} from the \specref{block}.

%Moon observes that this is no different than for any other form:
%
% Generally, \term{special forms} will return normally, \term{yielding}
% one or more values, but some \term{special forms} may cause a non-local exit.

The set of \term{special operator} \term{names} is fixed in \clisp; 
no way is provided for the user to define a \term{special operator}.
\Thenextfigure\ lists all of the \clisp\ \term{symbols}
that have definitions as \term{special operators}.

\DefineFigure{CLSpecialOps}
%% 5.1.3 2
%% 7.0.0 5
%% 7.7.0 1
%% 7.8.3 3
%% 7.8.3 5
%% 7.8.5 3
%% 7.10.0 1
\issue{COMPILER-LET-CONFUSION:ELIMINATE}
\issue{WITH-ADDED-METHODS:DELETE}
\issue{LOAD-TIME-EVAL:R**2-NEW-SPECIAL-FORM}
\issue{GENERIC-FLET-POORLY-DESIGNED:DELETE}
\displaythree{Common Lisp Special Operators}{
block&let*&return-from\cr
catch&load-time-value&setq\cr
eval-when&locally&symbol-macrolet\cr
flet&macrolet&tagbody\cr
function&multiple-value-call&the\cr
go&multiple-value-prog1&throw\cr
if&progn&unwind-protect\cr
labels&progv&\cr
let&quote&\cr
}
%Removed COMPILER-LET -kmp 3-Jan-91
%Removed WITH-ADDED-METHODS -kmp 7-Jan-91
%Removed DECLARE per Moon's comments. -kmp 10-Mar-91
%Added LOAD-TIME-VALUE -kmp 26-May-91
%Added LOCALLY per Barrett comments. -kmp 13-Oct-91
\endissue{GENERIC-FLET-POORLY-DESIGNED:DELETE}
\endissue{LOAD-TIME-EVAL:R**2-NEW-SPECIAL-FORM}
\endissue{WITH-ADDED-METHODS:DELETE}
\endissue{COMPILER-LET-CONFUSION:ELIMINATE}

%% 5.1.3 9
\endsubsubsubsubsection%{Special Forms}

\beginsubsubsubsubsection{Macro Forms}

If the \term{operator} names a \term{macro},
its associated \term{macro function} is applied
to the entire \term{form} and the result of that application is
used in place of the original \term{form}.

Specifically, a \term{symbol} names a \term{macro} in a given \term{lexical environment} if
\funref{macro-function} is \term{true} of the 
\term{symbol} and that \term{environment}.
The \term{function} returned by \funref{macro-function}
is a \term{function} of two arguments, called the
expansion function.
The expansion function is invoked by calling the \term{macroexpand hook} with
      the expansion function as its first argument,
      the entire \term{macro form} as its second argument,
%   and the current \term{lexical environment} 
% %% KAB thinks this is a bogus distinction.
% %       (or with the current compilation \term{environment},
% %        if the \term{form} is being processed by \funref{compile-file})
%% Trying again... -kmp&kab 8-Feb-92
    and an \term{environment object} (corresponding to the current \term{lexical environment})
      as its third argument.
The \term{macroexpand hook}, in turn, calls the expansion function with the
\term{form} as its first argument and the \term{environment} as its second argument.
The \term{value} of the expansion function, which is passed through
by the \term{macroexpand hook}, is a \term{form}. 
% (\funref{macroexpand}
% and \funref{macroexpand-1} attach a second, \term{generalized boolean} return value
% which is always \term{true} in this context, and hence always ignored.)
The returned \term{form} is \term{evaluated} in place of the original \term{form}.

\issue{SELF-MODIFYING-CODE:FORBID}
The consequences are undefined if a \term{macro function} destructively modifies
any part of its \term{form} argument.
\endissue{SELF-MODIFYING-CODE:FORBID}

%% 8.0.0 6
A \term{macro name} is not a \term{function designator},
and cannot be used as the \param{function} argument to \term{functions} 
such as \funref{apply}, \funref{funcall}, or \funref{map}.

An \term{implementation} is free to implement a \clisp\ \term{special operator}
as a \term{macro}.  An \term{implementation} is free to implement any
\term{macro} \term{operator} as a \term{special operator}, but only
if an equivalent definition of the \term{macro} is also provided.
% I don't think this adds anything to the above, and it's more confusing.
% So I commented it out.  --sjl 3 Mar 92
% %% 7.1.1 23
% A \clisp\ \term{symbol} may name both a \term{special operator} 
% and a \term{macro},
% %I added this: -kmp -kmp 12-Mar-91
% but in that case the definitions must be semantically equivalent.

\Thenextfigure\ lists some \term{defined names} that are applicable
to \term{macros}.

\displaythree{Defined names applicable to macros}{
*macroexpand-hook*&macro-function&macroexpand-1\cr
defmacro&macroexpand&macrolet\cr
}
%DEFINE-COMPILER-MACRO and COMPILER-MACRO-FUNCTION removed per Moon. -kmp 10-Mar-91

\endsubsubsubsubsection%{Macro Forms}

\beginsubsubsubsubsection{Function Forms}
\DefineSection{FunctionForms}

If the \term{operator} is a \term{symbol} naming a \term{function},
the \term{form} represents a \term{function form},
and the \term{cdr} of the list contains the \term{forms} 
which when evaluated will supply the arguments passed to the \term{function}.

When a \term{function name} is not defined, 
an error \oftype{undefined-function} should be signaled at run time;
\seesection\SemanticConstraints.

A \term{function form} is evaluated as follows:

The \term{subforms} in the \term{cdr} of the original \term{form}
are evaluated in left-to-right order in the current lexical and 
dynamic \term{environments}.  The \term{primary value} of each
such \term{evaluation} becomes an \term{argument} to the named \term{function};
% removed inappropriate reference to Lambda List section.  --sjl 3 Mar 92
any additional \term{values} returned by the \term{subforms} are discarded.

The \term{functional value} of the \term{operator} 
is retrieved from the \term{lexical environment},
and that \term{function} is invoked with the indicated arguments.

%%How the function treats its arguments, what bindings it establishes, etc. 
%% does not belong here. -kmp,moon
% and a new \term{lexical environment} is created that contains only
% the \term{bindings} established by \term{binding} each parameter
% name in the lambda list of the \term{function} with the
% corresponding value in the evaluated arguments along with a binding of
% the name of the \term{function} with the \term{exit point} that
% returns from the \term{function}. This is the lexical 
% \term{environment} used for evaluating the body of the \term{function}.

\issue{FUNCTION-CALL-EVALUATION-ORDER:MORE-UNSPECIFIED}
Although the order of \term{evaluation} of 
the \term{argument} \term{subforms} themselves is 
strictly left-to-right, it is not specified whether 
the definition of the \term{operator} in a \term{function form} is looked up 
before the \term{evaluation} of the \term{argument} \term{subforms},
after the \term{evaluation} of the \term{argument} \term{subforms},
or between the \term{evaluation} of any two \term{argument} \term{subforms} 
if there is more than one such \term{argument} \term{subform}.  
For example, the following might return 23 or~24.

\code
 (defun foo (x) (+ x 3))
 (defun bar () (setf (symbol-function 'foo) #'(lambda (x) (+ x 4))))
 (foo (progn (bar) 20))
\endcode
\endissue{FUNCTION-CALL-EVALUATION-ORDER:MORE-UNSPECIFIED}

A \term{binding} for a \term{function name} can be \term{established} in 
one of several ways.  A \term{binding} for a \term{function name} in 
the \term{global environment} can be \term{established} by 
 \macref{defun},
 \SETFof{fdefinition},
% added symbol-function --sjl 3 Mar 92
 \SETFof{symbol-function},
%% KAB: Doesn't belong here.
%\macref{defmacro},
%\SETFof{macro-function},
 \funref{ensure-generic-function},
 \macref{defmethod} (implicitly, due to \funref{ensure-generic-function}),
or
 \macref{defgeneric}.
A \term{binding} for a \term{function name} in the \term{lexical environment}
can be \term{established} by
   \specref{flet}
or \specref{labels}.
\issue{WITH-ADDED-METHODS:DELETE}
%\specref{with-added-methods},
\endissue{WITH-ADDED-METHODS:DELETE}%
\issue{GENERIC-FLET-POORLY-DESIGNED:DELETE}
%  \specref{generic-flet},
% or
%  \specref{generic-labels}.
\endissue{GENERIC-FLET-POORLY-DESIGNED:DELETE}

%%Moon thinks this is redundant, so asked that it be deleted. -kmp 11-Mar-91
% If a \term{symbol} that names a \term{function} appears
% as the first element of a function call \term{form}, then it refers
% to the definition established by the innermost
%  \specref{flet},
%  \specref{labels},
% \issue{WITH-ADDED-METHODS:DELETE}
% %\specref{with-added-methods},
% \endissue{WITH-ADDED-METHODS:DELETE}%
%  \specref{generic-flet},
% or
%  \specref{generic-labels}
% that textually contains the reference, or to the global definition (if any) if there
% is no such containing \term{form}.  As noted earlier, this use
% of \term{symbols} to name \term{functions} is
% independent of their use in naming dynamic and lexical variables.

\Thenextfigure\ lists some \term{defined names} that are applicable to \term{functions}.

\issue{GENERIC-FLET-POORLY-DESIGNED:DELETE}
\issue{WITH-ADDED-METHODS:DELETE}
\displaythree{Some function-related defined names}{
apply&fdefinition&mapcan\cr
call-arguments-limit&flet&mapcar\cr
complement&fmakunbound&mapcon\cr
constantly&funcall&mapl\cr
defgeneric&function&maplist\cr
defmethod&functionp&multiple-value-call\cr
defun&labels&reduce\cr
fboundp&map&symbol-function\cr
}
%Removed GENERIC-FLET, GENERIC-LABELS. -kmp 7-Feb-92
%Removed WITH-ADDED-METHODS -kmp 7-Jan-91
%Added MAPC, MAPCAR, MAPCAN, MAPLIST, MAPL, MAPCON, 
% CONSTANTLY, COMPLEMENT, REDUCE, FDEFINITION.
%Removed LAMBDA-LIST-KEYWORDS.
%Not including all the things with :TEST, :TEST-NOT, :KEY arguments, though.
%  -kmp 16-Oct-91
\endissue{WITH-ADDED-METHODS:DELETE}
\endissue{GENERIC-FLET-POORLY-DESIGNED:DELETE}

\endsubsubsubsubsection%{Function Forms}

\beginsubsubsubsubsection{Lambda Forms}
\DefineSection{LambdaForms}

A \term{lambda form} is similar to a \term{function form}, except that
the \term{function name} is replaced by a \term{lambda expression}.

A \term{lambda form} is equivalent to using \term{funcall} of a
\term{lexical closure} of the \term{lambda expression} on the given \term{arguments}.
(In practice, some compilers are more likely to produce inline code 
for a \term{lambda form} than for an arbitrary named function 
that has been declared \declref{inline}; however, such a difference
is not semantic.)

For further information, \seesection\LambdaExpressions.

\endsubsubsubsubsection%{Lambda Forms}

\endsubsubsubsection%{Conses as forms}

\beginsubsubsubsection{Self-Evaluating Objects}

\issue{EVAL-OTHER:SELF-EVALUATE}
A \term{form} that is neither a \term{symbol} nor a \term{cons} is 
defined to be a \term{self-evaluating object}.  \term{Evaluating}
such an \term{object} \term{yields} the \term{same} \term{object} 
as a result.
\endissue{EVAL-OTHER:SELF-EVALUATE}

Certain specific \term{symbols} and \term{conses} might also happen 
to be ``self-evaluating'' but only as a special case of a more 
general set of rules for the \term{evaluation} of \term{symbols} and
\term{conses}; such \term{objects} are not considered to be
\term{self-evaluating objects}.

% added  --sjl 3 Mar 92
The consequences are undefined if \term{literal objects} (including
\term{self-evaluating objects}) are destructively modified.

\beginsubsubsubsubsection{Examples of Self-Evaluating Objects}

\term{Numbers}, \term{pathnames}, and \term{arrays} are examples of
\term{self-evaluating objects}.

\code
 3 \EV 3
 #c(2/3 5/8) \EV #C(2/3 5/8)
 #p"S:[BILL]OTHELLO.TXT" \EV #P"S:[BILL]OTHELLO.TXT"
 #(a b c) \EV #(A B C)
 "fred smith" \EV "fred smith"
\endcode

\endsubsubsubsubsection%{Examples of Self-Evaluating Objects}

\endsubsubsubsection%{Self-Evaluating Objects}

\endsubsubsection%{Form Evaluation}

\endSubsection%{The Evaluation Model}

\beginsubsection{Lambda Expressions}
\DefineSection{LambdaExpressions}

% I found the last paragraph of this section very confusing, so I
% reworded the first paragraph to make it unnecessary.  -- sjl 3 Mar 92

% In a \term{lambda expression},
% the current lexical \term{environment} is extended by adding the \term{binding} of
% each \term{parameter} in the \term{lambda list} of the \term{lambda expression}
% with the corresponding \term{value} from the \term{arguments}.
% This is the \term{lexical environment} used for evaluating the body
% of the \term{lambda expression}.

In a \term{lambda expression},
the body is evaluated in a lexical \term{environment} that is formed by
adding the \term{binding} of 
each \term{parameter} in the \term{lambda list}
with the corresponding \term{value} from the \term{arguments}
to the current lexical \term{environment}.

For further discussion of how \term{bindings} are \term{established} 
based on the \term{lambda list}, \seesection\LambdaLists.

The body of a \term{lambda expression} is an \term{implicit progn};
the \term{values} it returns are returned by the \term{lambda expression}.

% When the \term{function} returns,
% the \term{lexical environment} that was in effect
% when the original \term{form} was evaluated 
% is re-established.

\endsubsection%{Lambda Expressions}

\beginSubsection{Closures and Lexical Binding}
 
%% 7.1.1 5
A \term{lexical closure} is a \term{function} that can refer to and alter
the values of \term{lexical bindings} \term{established} by \term{binding} \term{forms}
that textually include the function definition.
 
Consider this code, where \f{x} is not declared \declref{special}:
 
\code
 (defun two-funs (x)
   (list (function (lambda () x))
         (function (lambda (y) (setq x y)))))
 (setq funs (two-funs 6))
 (funcall (car funs)) \EV 6
 (funcall (cadr funs) 43) \EV 43
 (funcall (car funs)) \EV 43
\endcode

\Thespecform{function} coerces a 
\term{lambda expression} into a \term{closure} in which the 
\term{lexical environment} in effect when the \term{special form} is
evaluated is captured along with the \term{lambda expression}.

The function \f{two-funs} returns a \term{list} of two 
\term{functions}, each of which refers to the \term{binding} of the
variable \f{x} created on entry to the function \f{two-funs} when it
was called.
%% For Moon.
%with argument \f{6}.
This variable has the value \f{6}
initially, but \specref{setq} can alter this \term{binding}.
% %Barrett finds the next sentence confusing.
% %Moon, too.
% The \term{lexical closure} created for the first 
% \term{lambda expression} does not capture the value \f{6} for \f{x}
% when the \term{closure} is created.  The second \term{function} can be
% used to alter the variable binding (to \f{43}, in the example), and
% this altered variable binding then becomes accessible to the first
% \term{function}.
The \term{lexical closure} created for the first 
\term{lambda expression} does not ``snapshot'' the \term{value} \f{6} for \f{x}
when the \term{closure} is created; rather it captures the \term{binding} of \f{x}.
The second \term{function} can be used to alter the \term{value} in the same (captured)
\term{binding} (to \f{43}, in the example), and
this altered variable binding then affects the value returned by the first \term{function}.

                     
%% 7.1.1 6
In situations where a \term{closure} of a 
\term{lambda expression} over the same set of \term{bindings} may be
produced more than once, the various resulting \term{closures} may
or may not be \term{identical}, at the discretion of the \term{implementation}.
That is, two \term{functions} that are behaviorally
indistinguishable might or might not be \term{identical}.
Two \term{functions} that are behaviorally distinguishable are \term{distinct}.
For example:
 
\code
 (let ((x 5) (funs '()))
   (dotimes (j 10)                          
     (push #'(lambda (z)                        
               (if (null z) (setq x 0) (+ x z)))
           funs))
   funs)
\endcode
The result of the above \term{form} is a \term{list} of ten \term{closures}.
Each requires only the \term{binding} of \f{x}.
It is the same \term{binding} in each case, 
but the ten \term{closure} \term{objects} might or might not be \term{identical}.
On the other hand, the result of the \term{form}
 
\code
 (let ((funs '()))     
   (dotimes (j 10)
     (let ((x 5))
       (push (function (lambda (z)
                        (if (null z) (setq x 0) (+ x z))))
             funs)))
  funs)
\endcode
is also a \term{list} of ten \term{closures}.
However, in this case no two of the \term{closure} \term{objects} can
be \term{identical} because each \term{closure} is closed over a distinct
\term{binding} of \f{x}, and these \term{bindings} can be behaviorally
distinguished because of the use of \specref{setq}.
 
%% 7.1.1 7                        
The result of the \term{form}
 
\code
 (let ((funs '()))
   (dotimes (j 10)
     (let ((x 5))
       (push (function (lambda (z) (+ x z)))
            funs)))
   funs)
\endcode
is a \term{list} of ten \term{closure} \term{objects} that
might or might not be \term{identical}.
A different \term{binding} of \f{x} is involved for
each \term{closure}, but the \term{bindings} cannot be distinguished
because their values are the \term{same} and immutable (there being no occurrence
of \specref{setq} on \f{x}).  A compiler could internally
transform the \term{form} to
 
\code
 (let ((funs '()))
   (dotimes (j 10)
     (push (function (lambda (z) (+ 5 z)))
           funs))
  funs)
\endcode
where the \term{closures} may be \term{identical}.
%% Moon thought this was redundant and distracting. -kmp 8-Feb-92
% In some \term{implementations},
% two distinct evaluations of the same \specref{function} \term{form} might 
% produce \term{identical} \term{closures} if they must be behaviorally
% identical with respect to invocation.
 
%% 7.1.1 9
It is possible that a \term{closure} does not
close over any variable bindings.
In the code fragment
 
\code
 (mapcar (function (lambda (x) (+ x 2))) y)
\endcode
the function \f{(lambda (x) (+ x 2))} contains no references to any outside
object. In this case, the same \term{closure} might be returned
for all evaluations of the \specref{function} \term{form}.

\endSubsection%{Closures and Lexical Binding}
 
\beginSubsection{Shadowing}
\DefineSection{Shadowing}
%% 5.1.3 10
%% 3.0.0 13
%
% If two \term{forms} that establish lexical entities with the same
% \term{name} are textually nested, then references within the inner
% \term{form} refer to the entity established by the inner one; the
% inner one shadows the outer one.  Outside the inner \term{form} but
% inside the outer one, references refer to the entity established by
% the outer \term{form}.  For example:
%
%% Rewritten for Sandra:

If two \term{forms} that \term{establish} \term{lexical bindings} with
the same \term{name} $N$ are textually nested, then references to $N$
within the inner \term{form} refer to the \term{binding} established by
the inner \term{form}; the inner \term{binding} for $N$
\newtermidx{shadows}{shadow} the outer \term{binding} for $N$.  Outside the inner
\term{form} but inside the outer one, references to $N$ refer to the
\term{binding} established by the outer \term{form}.  For example:

%!!! Barrett thinks this doesn't explain, and that we should discuss,
%    (let ((x ...)) (let ((x ...)) (declare (special x)) ...))

\code
 (defun test (x z)
   (let ((z (* x 2)))
     (print z))
   z)
\endcode
The \term{binding} of the variable \f{z} by
\specref{let} shadows
the \term{parameter} binding for the function \f{test}.  The reference to the
variable \f{z} in the \funref{print} \term{form} refers to the \specref{let} binding.
The reference to \f{z} at the end of the function \f{test} 
refers to the \term{parameter} named \f{z}.

%!!! What to do with this use of "construct"? -kmp 3-Sep-91
%% 3.0.0 26
Constructs that are lexically scoped act as if new names were
generated for each \term{object} on each execution.  Therefore,
dynamic shadowing cannot occur.  For example:

\code
 (defun contorted-example (f g x)
   (if (= x 0)
       (funcall f)
       (block here
          (+ 5 (contorted-example g
                                  #'(lambda () (return-from here 4))
                                  (- x 1))))))
\endcode
Consider the call \f{(contorted-example nil nil 2)}.  This produces
\f{4}.  During the course of execution, there are three
calls to \f{contorted-example}, interleaved with two 
blocks:

\code
 (contorted-example nil nil 2)
   (block here\ssso ...)
     (contorted-example nil #'(lambda () (return-from here\ssso 4)) 1)
       (block here\ssst ...)
         (contorted-example #'(lambda () (return-from here\ssso 4))
                            #'(lambda () (return-from here\ssst 4))
                            0)
             (funcall f)
                    where f \EV #'(lambda () (return-from here\ssso 4))
                 (return-from here\ssso 4)
\endcode
At the time the \f{funcall} is executed
there are two \specref{block} \term{exit points} outstanding, each apparently
named \f{here}.
The \specref{return-from} \term{form} executed as a result of the \f{funcall}
operation
refers to the outer outstanding \term{exit point}
(here\ssso), not the
inner one (here\ssst).
It
refers to that \term{exit point} textually visible at the point of
execution of \specref{function}
(here abbreviated by the \f{\#'} syntax) that resulted
in creation of the \term{function} \term{object} actually invoked by 
\funref{funcall}.                       

%% 3.0.0 27
If, in this example, one were to change the \f{(funcall f)} to
\f{(funcall g)}, then the value of the call \f{(contorted-example nil nil 2)}
would be \f{9}.  The value would change because 
\funref{funcall} would cause the
execution of \f{(return-from here\ssst\ 4)}, thereby causing
a return from the inner \term{exit point} (here\ssst).
When that occurs, the value \f{4} is returned from the
middle invocation of \f{contorted-example}, \f{5} is added to that
to get \f{9}, and that value is returned from the outer block
and the outermost call to \f{contorted-example}.  The point
is that the choice of \term{exit point}
returned from has nothing to do with its
being innermost or outermost; rather,
it depends on the lexical environment
that is packaged up with a \term{lambda expression} when
\specref{function} is executed.
                                  
\endSubsection%{Shadowing}

\beginSubsection{Extent}
%% 3.0.0 28
\f{Contorted-example} works only because the
\term{function} named by \f{f} is invoked during the \term{extent} of the 
\term{exit point}.
Once the flow of execution has left the block,
the \term{exit point} is \term{disestablished}.  For example:

\code
 (defun invalid-example ()
   (let ((y (block here #'(lambda (z) (return-from here z)))))
     (if (numberp y) y (funcall y 5))))
\endcode
One might expect the call \f{(invalid-example)} to produce \f{5}
by the following incorrect reasoning:
\specref{let} binds \f{y} to the
value of \specref{block}; this value is a \term{function} resulting
from the \term{lambda expression}.  Because \f{y} is not a number, it is
invoked on the value \f{5}.  The \specref{return-from} should then
return this value from the                      
\term{exit point} named \f{here}, thereby
exiting from the block again and giving \f{y} the value \f{5}
which, being a number, is then returned as the value of the call
to \f{invalid-example}.

%% 3.0.0 29
The argument fails only because \term{exit points} have 
\term{dynamic extent}.  The argument is correct up to the execution of
\specref{return-from}.  The execution of \specref{return-from}
should signal an error \oftype{control-error}, however, not
because it cannot refer to the \term{exit point}, but because it
does correctly refer to an \term{exit point} and that 
\term{exit point} has been \term{disestablished}.

%% 3.0.0 14
%% 3.0.0 16
A reference by name to a dynamic \term{exit point} binding such as
a \term{catch tag} refers to the most recently 
\term{established} \term{binding} of that name that has not been 
\term{disestablished}.  For example:

\code
 (defun fun1 (x)
   (catch 'trap (+ 3 (fun2 x))))
 (defun fun2 (y)
   (catch 'trap (* 5 (fun3 y))))
 (defun fun3 (z)
   (throw 'trap z))
\endcode
Consider the call \f{(fun1 7)}.  The result is \f{10}.  At the time
the \specref{throw} is executed, there are two outstanding catchers with the
name \f{trap}: one established within procedure \f{fun1}, and the other
within procedure \f{fun2}.  The latter is the more recent, and so the
value \f{7} is returned from \specref{catch} in \f{fun2}.
Viewed from within \f{fun3}, the \specref{catch} 
in \f{fun2} shadows the one in \f{fun1}.
Had \f{fun2} been defined as

\code
 (defun fun2 (y)
   (catch 'snare (* 5 (fun3 y))))
\endcode
then the two \term{exit points} 
would have different \term{names}, and therefore the one
in \f{fun1} would not be shadowed.  The result would then have been \f{7}.

\endSubsection%{Extent}

\beginSubsection{Return Values}

Ordinarily the result of calling a \term{function} is a single \term{object}.
Sometimes, however, it is convenient for a function to compute several
\term{objects} and return them.

% slight rewording -- sjl 3 Mar 92
In order to receive other than exactly one value from a \term{form},
one of several \term{special forms} or \term{macros} must be used to request those
values.  If a \term{form} produces \term{multiple values} which were not
requested in this way, then the first value is given to the caller and
all others are discarded; if the \term{form} produces zero values,
then the caller receives \nil\ as a value.

\Thenextfigure\ lists 
some \term{operators} for receiving \term{multiple values}\meaning{2}.
These \term{operators} can be used to specify 
    one or more \term{forms} to \term{evaluate} 
and where to put the \term{values} returned by those \term{forms}.

\displaythree{Some operators applicable to receiving multiple values}{
multiple-value-bind&multiple-value-prog1&return-from\cr
multiple-value-call&multiple-value-setq&throw\cr
multiple-value-list&return&\cr
}

\Thefunction{values} can produce \term{multiple values}\meaning{2}.
\f{(values)} returns zero values;
\f{(values \param{form})} returns the \term{primary value} returned by \param{form};
\f{(values \param{form1} \param{form2})} returns two values,
    the \term{primary value} of \param{form1}
and the \term{primary value} of \param{form2};
and so on.

See \conref{multiple-values-limit} and \funref{values-list}.

%% This little piece of language analysis isn't useful to the spec.
%% -kmp&kab 13-Oct-91
% 
% \term{Multiple values} can result from a \term{form} under precisely these
% circumstances:
% \beginlist
% \itemitem{\b{Evaluation and Application}}
% 
% \beginlist
% \itemitem{\funref{eval}}
% 
% \funref{eval} returns \term{multiple values} if the form given it to
% evaluate produces \term{multiple values}.
% 
% \itemitem{\funref{apply}, \funref{funcall}, \specref{multiple-value-call}}
% 
% \funref{apply}, \funref{funcall}, and \specref{multiple-value-call}
% can return \term{multiple values} from the function applied or called.
% \endlist
% 
% \itemitem{\b{Sequential Forms}}
% 
% \beginlist
% \itemitem{\specref{progn}}
% 
% The special form \specref{progn} can return \term{multiple values} resulting
% from evaluation of the last \term{subform}.
% 
% \itemitem{\term{implicit progn}}
% 
% Other situations are referred to as an \term{implicit progn}, in
% which several forms are evaluated and the results of all but the last
% form are discarded.  An \term{implicit progn} returns multiple
% values if the last form does.  These situations include the body of a
% \term{lambda expression}, including those constructed implicitly by
% \macref{defmethod}, \macref{defun}, \macref{defmacro},
% \issue{DEFINE-COMPILER-MACRO:X3J13-NOV89}
% \macref{define-compiler-macro},
% \endissue{DEFINE-COMPILER-MACRO:X3J13-NOV89}
% \specref{flet},
% \specref{macrolet}, \specref{generic-flet}, \specref{labels},
% \specref{generic-labels}, \macref{deftype}.  Also included are
% bodies of the \specref{eval-when}, \specref{progv},
% \specref{let}, \specref{let*}, \macref{when},
% \macref{unless}, \specref{block},
% \macref{multiple-value-bind}, and \specref{catch} \term{forms}, as well as
% clauses in such conditional \term{forms} as \macref{case},
% \macref{typecase}, \macref{ecase}, \macref{etypecase},
% \macref{ccase}, and \macref{ctypecase}. If a body has no
% \term{forms}, a single \nil\ is returned.
% \endlist
% 
% \itemitem{\b{Conditional Forms}}
% 
% \beginlist
% \itemitem{\specref{if}}
% 
% \specref{if} can return \term{multiple values} from whichever \term{subform} is selected
% (the \i{then} form or the \i{else} form).
% 
% \itemitem{\macref{and}, \macref{or}}
% 
% \macref{and} and \macref{or} can return \term{multiple values} from the last \term{subform}
% but not from \term{subforms} other than the last.
% 
% \itemitem{\macref{cond}}
% 
% \macref{cond} can return \term{multiple values} from the last \term{subform} of
% the \term{implicit progn} of the selected clause.
% If, however, the clause selected is a singleton clause,
% then only a single value (the \term{true} value returned by the predicate)
% is returned.  This is true even if the singleton clause is
% the last clause of the \macref{cond}.  A final clause
% {\tt (\i{form})} is not equivalent to {\tt (t \i{form})}
% because the latter returns \term{multiple values} from \i{form}
% while the former does not; a final clause of {\tt (\i{form})}
% is instead equivalent to {\tt (t (values \i{forms}))}.
% \endlist
% 
% \itemitem{\b{Returning from a Block}}
% 
% \beginlist
% \itemitem{\specref{block}}
% 
% The \specref{block} special form can return \term{multiple values} 
% from its last \term{subform} when it exits normally.  
% If \specref{return-from} (or \macref{return}) is used to terminate
% the \term{block} prematurely, then \specref{return-from} can
% return \term{multiple values} 
% %from its \term{subform} 
% as the \term{values} of the terminated \term{block}.
% Other \term{operators} that implicitly create \term{blocks},
% such as \macref{do}, \macref{dolist}, \macref{dotimes}, 
%         \macref{prog}, and \macref{prog*},
% also can return \term{multiple values} specified by
% \specref{return-from} (or \macref{return}).
% 
% \itemitem{\macref{do}}
% 
% \macref{do} can return \term{multiple values} from the last form of the
% exit clause, exactly as if the exit clause were a \macref{cond}
% clause.  Similarly, \macref{dolist} and \macref{dotimes} can
% return \term{multiple values} from the \param{result-form} if that is
% executed.  \macref{loop} can return \term{multiple values} by using
% \macref{return} and \specref{return-from}.  These situations are
% all examples of implicit uses of \specref{return-from}.
% 
% \endlist
% 
% \itemitem{\b{Throwing out of a Catch}}
% 
% \beginlist
% 
% \itemitem{\specref{catch}}
% 
% The \specref{catch} \term{form} returns \term{multiple values} if
% the result form in a \specref{throw} exiting from
% such a catch produces \term{multiple values}.
% \endlist
% 
% \itemitem{\b{Miscellaneous Situations}}
% 
% \beginlist
% 
% \itemitem{\macref{multiple-value-prog1}}
% 
% \specref{multiple-value-prog1} can return \term{multiple values} from its first
% \term{subform}.  \macref{prog1} always returns a single value.
% 
% \itemitem{\specref{unwind-protect}}
% 
% \specref{unwind-protect} returns \term{multiple values} if the form it protects
% returns \term{multiple values}.
% 
% \itemitem{\specref{the}}
% 
% \specref{the} returns \term{multiple values} if the form it contains returns
% \term{multiple values}.
% \endlist
% \endlist
% 
% Among special forms that never return \term{multiple values} are
% \specref{setq}, \macref{multiple-value-setq}, \macref{prog1}, and
% \macref{prog2}.
% 
% In an ordinary function call,
% each argument form produces exactly one argument;  if such a form
% returns zero values, \nil\ is used for the argument, and if more than one
% value, all but the first are discarded.
% Similarly, conditional \term{forms} (such as \specref{if} \term{forms})
% that test the value of a form
% will use exactly one value, the first one, from that form and discard the rest;
% such \term{forms} will use \nil\ as the test value if zero values are returned.
%          
\endSubsection%{Return Values}