bugfix in xsymbols_output

--- a/src/Pure/proof_general.ML	Wed Apr 14 15:09:51 2004 +0200
+++ b/src/Pure/proof_general.ML	Thu Apr 15 09:33:12 2004 +0200
@@ -48,7 +48,7 @@
 fun xsymbols_output s =
   if xsymbolsN mem_string ! print_mode andalso exists_string (equal "\\") s then
     let val syms = Symbol.explode s
-    in (implode (map Symbol.escape syms), real (Symbol.length syms)) end
+    in (implode (map (fn "\\" => "\\\\" | c => c) syms), real (Symbol.length syms)) end
   else (s, real (size s));
 
 fun pgml_output (s, len) =