Issue most easily explained with a simple reproduction:

guile> (use-syntax (ice-9 syncase))
guile> (@ (guile) car)
ERROR: invalid syntax #<variable b7c50a40 value: #<primitive-procedure car>>

The reasoning of the fix is best described in the long comment in the
patch below.  If you would rather that I modify psyntax.ss to use a new
procedure name rather than `self-evaluating?' please let me know.  Patch
does not include log entries, listed immediately below instead:

ice-9/ChangeLog:

2007-12-14  Stephen Compall  <s11@member.fsf.org>

	* syncase.scm: Support datums where (variable? DATUM) => #t
	when syncase transformer is installed.

test-suite/ChangeLog:

2007-12-14  Stephen Compall  <s11@member.fsf.org>

	* tests/syncase.test: Test that @-forms can be expanded by
	the syncase transformer.


Index: ice-9/syncase.scm
===================================================================
RCS file: /sources/guile/guile/guile-core/ice-9/syncase.scm,v
retrieving revision 1.35
diff -u -d -u -r1.35 syncase.scm
--- ice-9/syncase.scm	16 Apr 2006 23:43:48 -0000	1.35
+++ ice-9/syncase.scm	14 Dec 2007 07:55:49 -0000
@@ -194,6 +194,18 @@
               "syncase's gensym expected 0 or 1 arguments, got "
               (length rest)))))))))
 
+;; The question syncase is asking when it asks `self-evaluating?' is
+;; not really whether a given datum is self-evaluating, but whether it
+;; should pass the datum through as-is to the evaluator.  With that in
+;; mind, we wrap core `self-evaluating?' here so that syncase will
+;; pass-through variables, meaning (variable? datum) => #t, to the
+;; evaluator.  Without this, the system transformer will see a
+;; variable datum as invalid syntax, which it is not to the core
+;; transformer.
+(define (self-evaluating? datum)
+  (or ((@ (guile) self-evaluating?) datum)
+      (variable? datum)))
+
 ;;; Load the preprocessed code
 
 (let ((old-debug #f)
Index: test-suite/tests/syncase.test
===================================================================
RCS file: /sources/guile/guile/guile-core/test-suite/tests/syncase.test,v
retrieving revision 1.5
diff -u -d -u -r1.5 syncase.test
--- test-suite/tests/syncase.test	16 Apr 2006 23:27:14 -0000	1.5
+++ test-suite/tests/syncase.test	14 Dec 2007 07:55:49 -0000
@@ -34,3 +34,15 @@
 
 (pass-if "basic syncase macro"
   (= (plus 1 2 3) (+ 1 2 3)))
+
+(pass-if "variable?s recognized as datums"
+  (false-if-exception
+   (begin (eq? car (eval '(@ (guile) car) (current-module)))
+	  #t)))
+
+(define-syntax export-safe-plus
+  (syntax-rules ()
+    ((_ x ...) ((@ (guile) +) x ...))))
+
+(pass-if "variable?s passed through to evaluator"
+  (= (export-safe-plus 1 2 3) (+ 1 2 3)))


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