fixed witness syntax;

--- a/doc-src/Ref/theory-syntax.tex	Wed May 07 16:38:33 1997 +0200
+++ b/doc-src/Ref/theory-syntax.tex	Wed May 07 16:40:00 1997 +0200
@@ -127,7 +127,7 @@
 instance : 'instance' ( name '<' name | name '::' arity) witness
          ;
 
-witness : (() | ((string | longident) + ',')) (() | verbatim)
+witness : (() | '(' ((string | longident) + ',') ')') (() | verbatim)
         ;
 
 oracle : 'oracle' name