code: func (f Fields) Copy(other Fields) Fields {
  	if len(f) == 0 && len(other) == 0 {
  		return nil
  	}
  
  	ret := make(Fields, len(f)+len(other))
  	for k, v := range f {
  		ret[k] = v
  	}
  	for k, v := range other {
  		ret[k] = v
  	}
  	return ret
  }
docs: // Copy returns a copy of this Fields with the keys from other overlaid on top
  // of this one's.