So apparently try-/all-/test-completion(s) functions test if COLLECTION
is a function imprecisely. I propose we fix these functions. Objections?

=== modified file 'src/minibuf.c'
--- src/minibuf.c	2013-11-06 04:11:04 +0000
+++ src/minibuf.c	2013-11-14 05:23:53 +0000
@@ -1199,9 +1199,7 @@
     type = (HASH_TABLE_P (collection) ? hash_table
 	    : VECTORP (collection) ? obarray_table
 	    : ((NILP (collection)
-		|| (CONSP (collection)
-		    && (!SYMBOLP (XCAR (collection))
-			|| NILP (XCAR (collection)))))
+		|| (CONSP (collection) && !FUNCTIONP (collection)))
 	       ? list_table : function_table));
   ptrdiff_t idx = 0, obsize = 0;
   int matchcount = 0;
@@ -1460,9 +1458,7 @@
   Lisp_Object allmatches;
   int type = HASH_TABLE_P (collection) ? 3
     : VECTORP (collection) ? 2
-    : NILP (collection) || (CONSP (collection)
-			    && (!SYMBOLP (XCAR (collection))
-				|| NILP (XCAR (collection))));
+    : NILP (collection) || (CONSP (collection) && !FUNCTIONP (collection));
   ptrdiff_t idx = 0, obsize = 0;
   ptrdiff_t bindcount = -1;
   Lisp_Object bucket, tem, zero;
@@ -1691,9 +1687,7 @@
 
   CHECK_STRING (string);
 
-  if ((CONSP (collection)
-       && (!SYMBOLP (XCAR (collection)) || NILP (XCAR (collection))))
-      || NILP (collection))
+  if ((CONSP (collection) && !FUNCTIONP (collection)) || NILP (collection))
     {
       tem = Fassoc_string (string, collection, completion_ignore_case ? Qt : Qnil);
       if (NILP (tem))