code: private function parseOperationTypeDefinition()
      {
          $start     = $this->lexer->token;
          $operation = $this->parseOperationType();
          $this->expect(Token::COLON);
          $type = $this->parseNamedType();
  
          return new OperationTypeDefinitionNode([
              'operation' => $operation,
              'type'      => $type,
              'loc'       => $this->loc($start),
          ]);
      }
docs: @return OperationTypeDefinitionNode
  
  @throws SyntaxError