code: final public function addToSet($fieldName, array $values)
      {
          $this->guardFrozenMessage();
          $field = static::schema()->getField($fieldName);
          Assertion::true($field->isASet(), sprintf('Field [%s] must be a set.', $fieldName), $fieldName);
  
          foreach ($values as $value) {
              if (0 === strlen($value)) {
                  continue;
              }
              $field->guardValue($value);
              $key = strtolower(trim((string) $value));
              $this->data[$fieldName][$key] = $value;
          }
  
          if (!empty($this->data[$fieldName])) {
              unset($this->clearedFields[$fieldName]);
          }
  
          return $this;
      }
docs: {@inheritdoc}
  @return static