code: func (obj *Doc) CheckExpression(ctx context.Context, expr string, labels []string) (string, []*NxRange, []*NxRange, error) {
  	result := &struct {
  		ErrorMsg string	`json:"qErrorMsg"`
  		BadFieldNames []*NxRange	`json:"qBadFieldNames"`
  		DangerousFieldNames []*NxRange	`json:"qDangerousFieldNames"`
  	} {}
  	err := obj.rpc(ctx, "CheckExpression", result, expr, labels)
  	return result.ErrorMsg, result.BadFieldNames, result.DangerousFieldNames, err 
  }
docs: // Checks if a given expression is valid.
  // The expression is correct if the parameters qErrorMsg , qBadFieldNames and qDangerousFieldNames are empty.
  //
  // Parameters:
  //
  // ◾ expr     -   Expression to check.
  //
  // ◾ labels   -   List of labels.
  //