code: func (a *Attempt_ID) ToProperty() (datastore.Property, error) {
  	return datastore.MkPropertyNI(a.DMEncoded()), nil
  }
docs: // ToProperty implements datastore.PropertyConverter for the purpose of
  // embedding this Attempt_ID as the ID of a luci/gae compatible datastore
  // object. The numerical id field is stored as an inverted, hex-encoded string,
  // so that Attempt_ID{"quest", 1} would encode as "quest|fffffffe". This is done
  // so that the __key__ ordering in the dm application prefers to order the most
  // recent attempts first.
  //
  // The go representation will always have the normal non-flipped numerical id.