code: def parse_type_system_extension(lexer: Lexer) -> TypeSystemExtensionNode:
      
      keyword_token = lexer.lookahead()
      if keyword_token.kind == TokenKind.NAME:
          func = _parse_type_extension_functions.get(cast(str, keyword_token.value))
          if func:
              return func(lexer)
      raise unexpected(lexer, keyword_token)
docs: TypeSystemExtension