xo-reader2: accept parsed expression at top level..

xo-reader2/src/reader2/ParserResult.cpp

@@ -36,6 +36,16 @@ namespace xo {
                   error_description_{error_description}
         {}
 
+        ParserResult
+        ParserResult::expression(std::string_view ssm_name,
+                                 obj<AExpression> expr)
+        {
+            return ParserResult(parser_result_type::expression,
+                                expr,
+                                ssm_name,
+                                nullptr);
+        }
+
         ParserResult
         ParserResult::error(std::string_view ssm_name,
                             const DString * errmsg)