Fixed problem with const initializer

Former-commit-id: bcd1343169cd095f07e2725a2baa24e8bcde3371

src/parser/ExpressionParser.h

@@ -203,7 +203,7 @@ namespace storm {
             qi::symbols<char, storm::expressions::Expression> const* identifiers_;
             
             // The symbol table of invalid identifiers.
-            qi::symbols<char, uint_fast64_t> const& invalidIdentifiers_;
+            qi::symbols<char, uint_fast64_t> invalidIdentifiers_;
             
             // Rules for parsing a composed expression.
             qi::rule<Iterator, storm::expressions::Expression(), Skipper> expression;