code: func KeyFrom(fields []string, doc Document) Key {
  	key := make(Key, 0, len(fields))
  	for _, f := range fields {
  		if s, ok := doc[f].(String); ok {
  			key = append(key, string(s))
  		}
  	}
  	return key
  }
docs: // KeyFrom extracts a set of fields as a Key from Document.