Redefine backend bugfix

Ignore-this: e25b20f012fbe14a22c5ff4113de9cd6

The "Redefine Backend" restart in REGISTER-BACKEND did not work as
advertised (it did not register the new backend at all). This patch
makes sure the old backend is removed and the new one is added.

darcs-hash:20100129161309-55f2f-d3e841cb2d573706d9936e213d664e02abf8cf9a.gz

backends.lisp

@@ -106,9 +106,10 @@ if ERRORP is false, otherwise an error is signalled."
                                  :old-magic-numbers old-magic-numbers
                                  :compatible-magic-numbers compatible-magic-numbers
                                  :stream-type  stream-type)))
-    (if (assoc name *registered-backends*)
-        (cerror "Redefine backend" "Backend ~A is already defined." name)
-        (push (cons name instance) *registered-backends*))
+    (when (assoc name *registered-backends*)
+      (cerror "Redefine backend" "Backend ~A is already defined." name)
+      (setf *registered-backends* (remove name *registered-backends* :key #'car)))
+    (push (cons name instance) *registered-backends*)
     instance))
 
 (defun alias-backend (old alias)