We can now use concept sets in BNF definitions

parsers/BaseParser.py

@@ -1,4 +1,7 @@
 from dataclasses import dataclass
+
+from core.builtin_concepts import BuiltinConcepts
+from core.concept import Concept
 from core.tokenizer import TokenKind, Keywords
 from core.sheerka_logger import get_logger
 import logging
@@ -83,6 +86,17 @@ class BaseParser:
             value = context.return_value_to_str(r)
             context.log(self.log, f"  Recognized '{value}'", self.name)
 
+    def get_return_value_body(self, sheerka, source, tree, try_parse):
+        if len(self.error_sink) == 1 and isinstance(self.error_sink[0], Concept):
+            return self.error_sink[0]
+
+        return sheerka.new(
+            BuiltinConcepts.PARSER_RESULT,
+            parser=self,
+            source=source,
+            body=self.error_sink if self.has_error else tree,
+            try_parsed=try_parse)
+
     @staticmethod
     def get_text_from_tokens(tokens, custom_switcher=None):
         if tokens is None: