code: func (b *Base) PickFuncIf(p Predicate, converter ValueConverter, keys ...string) Serializer {
  	b.modifiers = append(b.modifiers, func(m jsonMap) jsonMap {
  		if p(b.raw) {
  			for _, key := range keys {
  				m[key] = converter(b.reflected.FieldByName(key).Interface())
  			}
  		}
  		return m
  	})
  	return b
  }
docs: // PickFuncIf adds the given fields to the result after applying the converter if the predicate returns true