Add substitution in expression

larcs.org

@@ -186,6 +186,11 @@ Of the various types, there are three atomic types, ~<number>~, ~<variable>~ and
   (defmethod atomicp ((expression <atomic>))
     (declare (ignore expression))
     t)
+
+  (defmethod subsitute-expression (expression with (in <atomic>))
+    (if (eqal expression in)
+        with
+        in))
 #+END_SRC
 
 *** DONE Numbers