code: func (b *Base) AddFuncIf(p Predicate, key string, f ValueConverter) Serializer {
  	b.modifiers = append(b.modifiers, func(m jsonMap) jsonMap {
  		if p(b.raw) {
  			m[key] = f(b.raw)
  		}
  		return m
  	})
  	return b
  }
docs: // AddFuncIf adds a computed custom field to the result if the Predicate returns true