code: final public function addToMap($fieldName, $key, $value)
      {
          $this->guardFrozenMessage();
          $field = static::schema()->getField($fieldName);
          Assertion::true($field->isAMap(), sprintf('Field [%s] must be a map.', $fieldName), $fieldName);
  
          if (null === $value) {
              return $this->removeFromMap($fieldName, $key);
          }
  
          $field->guardValue($value);
          $this->data[$fieldName][$key] = $value;
          unset($this->clearedFields[$fieldName]);
  
          return $this;
      }
docs: {@inheritdoc}
  @return static