code: func (v delegatingStrategy) MergeMap(diff apply.MapElement) (apply.Result, error) {
  	switch diff.GetFieldMergeType() {
  	case apply.MergeStrategy:
  		return v.merge.MergeMap(diff)
  	case apply.ReplaceStrategy:
  		return v.replace.MergeMap(diff)
  	case apply.RetainKeysStrategy:
  		return v.retainKeys.MergeMap(diff)
  	default:
  		return v.merge.MergeMap(diff)
  	}
  }
docs: // MergeMap delegates visiting a map based on the field patch strategy.
  // Defaults to "merge"