code: func parseOperationType(parser *Parser) (string, error) {
  	operationToken, err := expect(parser, lexer.NAME)
  	if err != nil {
  		return "", err
  	}
  	switch operationToken.Value {
  	case ast.OperationTypeQuery:
  		return operationToken.Value, nil
  	case ast.OperationTypeMutation:
  		return operationToken.Value, nil
  	case ast.OperationTypeSubscription:
  		return operationToken.Value, nil
  	default:
  		return "", unexpected(parser, operationToken)
  	}
  }
docs: /**
   * OperationType : one of query mutation subscription
   */