org-export: Export snippets are not skipped automatically: back-ends decide

* contrib/lisp/org-export.el (org-export-snippet-backend): New
  function.
(org-export--skip-p): Remove automatic skip of export snippets.
* EXPERIMENTAL/org-e-ascii.el (org-e-ascii-export-snippet): Use new
  function.
* EXPERIMENTAL/org-e-html.el (org-e-html-export-snippet): Use new
  function.
* EXPERIMENTAL/org-e-latex.el (org-e-latex-export-snippet): Use new
  function.
* EXPERIMENTAL/org-e-odt.el (org-e-odt-export-snippet): Use new
  function.
* testing/contrib/lisp/test-org-export.el: Add test.

The idea behind this change is that a given back-end may decide to
keep an export snippet not directly targetted at it (i.e. a beamer
back-end that would want to keep latex snippets).  Hence, filtering
snippets is on back-ends side, and a new function is added to help
them in that task.

EXPERIMENTAL/org-e-ascii.el

@@ -1101,7 +1101,8 @@ CONTENTS is nil.  INFO is a plist holding contextual information."
 (defun org-e-ascii-export-snippet (export-snippet contents info)
   "Transcode a EXPORT-SNIPPET object from Org to ASCII.
 CONTENTS is nil.  INFO is a plist holding contextual information."
-  (org-element-property :value export-snippet))
+  (when (eq (org-export-snippet-backend export-snippet) 'e-ascii)
+    (org-element-property :value export-snippet)))
 
 
 ;;;; Export Block

EXPERIMENTAL/org-e-html.el

@@ -2187,7 +2187,8 @@ CONTENTS is nil.  INFO is a plist holding contextual information."
 (defun org-e-html-export-snippet (export-snippet contents info)
   "Transcode a EXPORT-SNIPPET object from Org to HTML.
 CONTENTS is nil.  INFO is a plist holding contextual information."
-  (org-element-property :value export-snippet))
+  (when (eq (org-export-snippet-backend export-snippet) 'e-html)
+    (org-element-property :value export-snippet)))
 
 
 ;;;; Export Block

EXPERIMENTAL/org-e-latex.el

@@ -972,7 +972,8 @@ CONTENTS is nil.  INFO is a plist holding contextual information."
 (defun org-e-latex-export-snippet (export-snippet contents info)
   "Transcode a EXPORT-SNIPPET object from Org to LaTeX.
 CONTENTS is nil.  INFO is a plist holding contextual information."
-  (org-element-property :value export-snippet))
+  (when (eq (org-export-snippet-backend export-snippet) 'e-latex)
+    (org-element-property :value export-snippet)))
 
 
 ;;;; Export Block

EXPERIMENTAL/org-e-odt.el

@@ -3911,7 +3911,8 @@ CONTENTS is nil.  INFO is a plist holding contextual information."
 (defun org-e-odt-export-snippet (export-snippet contents info)
   "Transcode a EXPORT-SNIPPET object from Org to HTML.
 CONTENTS is nil.  INFO is a plist holding contextual information."
-  (org-element-property :value export-snippet))
+  (when (eq (org-export-snippet-backend export-snippet) 'e-odt)
+    (org-element-property :value export-snippet)))
 
 
 ;;;; Export Block

contrib/lisp/org-export.el

@@ -1433,15 +1433,7 @@ OPTIONS is the plist holding export options."
      (or (not (plist-get options :with-drawers))
 	 (and (consp (plist-get options :with-drawers))
 	      (not (member (org-element-property :drawer-name blob)
-			   (plist-get options :with-drawers))))))
-    ;; Check export snippet.
-    (export-snippet
-     (let* ((raw-back-end (org-element-property :back-end blob))
-	    (true-back-end
-	     (or (cdr (assoc raw-back-end org-export-snippet-translation-alist))
-		 raw-back-end)))
-       (not (string= (symbol-name (plist-get options :back-end))
-		     true-back-end))))))
+			   (plist-get options :with-drawers))))))))
 
 
 
@@ -2421,6 +2413,24 @@ file should have."
 ;; macros, references, src-blocks, tables and tables of contents are
 ;; implemented.
 
+;;;; For Export Snippets
+
+;; Every export snippet is transmitted to the back-end.  Though, the
+;; latter will only retain one type of export-snippet, ignoring
+;; others, based on the former's target back-end.  The function
+;; `org-export-snippet-backend' returns that back-end for a given
+;; export-snippet.
+
+(defun org-export-snippet-backend (export-snippet)
+  "Return EXPORT-SNIPPET targeted back-end as a symbol.
+Translation, with `org-export-snippet-translation-alist', is
+applied."
+  (let ((back-end (org-element-property :back-end export-snippet)))
+    (intern
+     (or (cdr (assoc back-end org-export-snippet-translation-alist))
+	 back-end))))
+
+
 ;;;; For Footnotes
 
 ;; `org-export-collect-footnote-definitions' is a tool to list

testing/contrib/lisp/test-org-export.el

@@ -24,9 +24,6 @@
 
 ;;; Tests
 
-(require 'org-test)
-(require 'org-export)
-
 (defmacro org-test-with-backend (backend &rest body)
   "Execute body with an export back-end defined.
 
@@ -237,3 +234,16 @@ text
       (forward-line 3)
       (mark-paragraph)
       (should (equal (org-export-as 'test) "text\n")))))
+
+(ert-deftest test-org-export/export-snippet ()
+  "Test export snippets transcoding."
+  (org-test-with-temp-text "@test{A}@t{B}"
+    (org-test-with-backend "test"
+      (flet ((org-test-export-snippet
+	      (snippet contents info)
+	      (when (eq (org-export-snippet-backend snippet) 'test)
+		(org-element-property :value snippet))))
+	(let ((org-export-snippet-translation-alist nil))
+	  (should (equal (org-export-as 'test) "A\n")))
+	(let ((org-export-snippet-translation-alist '(("t" . "test"))))
+	  (should (equal (org-export-as 'test) "AB\n")))))))