Fix regression in Parser: failure to parse top level symset.

It was trying to parse a position instead.

libwccl/parser/Parser.cpp

@@ -176,7 +176,7 @@ boost::shared_ptr<ANTLRParserResultBase> Parser::parseAnyOperator(
 		ANTLRLexer lexer(ss);
 		ANTLRParser parser(lexer);
 		try {
-			result = parser.parse_position_operator(tagset_);
+			result = parser.parse_symset_operator(tagset_);
 		} catch (antlr::ANTLRException& e) {
 			errors << "(as tset) " << e.getMessage() << "\n";
 			// ignore, try another type