code: func Singlep(iter Enumerable, pred Predicate) (retval interface{}, err error) {
  	iter = Where(iter, pred)
  	return Single(iter)
  }
docs: // Singlep retrieces the only element from a list that matches a criteria. If
  // no match is found, or two or more are found, `Singlep` returns nil and an
  // error.