code: func (c *Call) FieldArg() (string, error) {
  	for arg := range c.Args {
  		if !IsReservedArg(arg) {
  			return arg, nil
  		}
  	}
  	return "", fmt.Errorf("no field argument specified")
  }
docs: // FieldArg determines which key-value pair contains the field and rowID,
  // in the case of arguments like Set(colID, field=rowID).
  // Returns the field as a string if present, or an error if not.