Trap ArgumentException when a user-defined token has the same name as a builtin token.

Author: Steve Ives

CodeGenEngine/Tokenizer.dbl

@@ -1866,7 +1866,16 @@ namespace CodeGen.Engine
 					newMeta.Name = ut.Name
 					newMeta.TypeOfToken = TokenType.User
 					newMeta.Validity = TokenValidity.Anywhere
-					addLookupToken(newMeta)
+					try
+					begin
+						addLookupToken(newMeta)
+					end
+					catch (e, @ArgumentException)
+					begin
+						;;User token name clashes with a "baked in" token.
+						reportError(String.Format("{0} may not be used as a user-defined token. A token with that name already exists!", ut.Name))
+					end
+					endtry
 				end
 			end
 		endmethod