code: private function parseObjectTypeExtension()
      {
          $start = $this->lexer->token;
          $this->expectKeyword('extend');
          $this->expectKeyword('type');
          $name       = $this->parseName();
          $interfaces = $this->parseImplementsInterfaces();
          $directives = $this->parseDirectives(true);
          $fields     = $this->parseFieldsDefinition();
  
          if (count($interfaces) === 0 &&
              count($directives) === 0 &&
              count($fields) === 0
          ) {
              throw $this->unexpected();
          }
  
          return new ObjectTypeExtensionNode([
              'name'       => $name,
              'interfaces' => $interfaces,
              'directives' => $directives,
              'fields'     => $fields,
              'loc'        => $this->loc($start),
          ]);
      }
docs: @return ObjectTypeExtensionNode
  
  @throws SyntaxError