fixed definitions of last clauses in cond and case macros.

scheme/last-revision

@@ -1 +1 @@
-1751
+1752

scheme/psyntax.expander.ss

@@ -1031,7 +1031,7 @@
     (lambda (e)
       (define (build-last cls)
         (syntax-match cls (else)
-          ((else e e* ...) `(begin ,e . ,e*))
+          ((else e e* ...) `(let () #f ,e . ,e*))
           (_ (build-one cls '(if #f #f)))))
       (define (build-one cls k)
         (syntax-match cls ()
@@ -1520,7 +1520,7 @@
              (cond
                ((null? cls*)
                 (syntax-match cls (else =>)
-                  ((else e e* ...) `(begin ,e . ,e*))
+                  ((else e e* ...) `(let () #f ,e . ,e*))
                   ((e => p) `(let ((t ,e)) (if t (,p t))))
                   ((e) `(or ,e (if #f #f)))
                   ((e e* ...) `(if ,e (begin . ,e*)))