query.go
func Singlep(iter Enumerable, pred Predicate) (retval interface{}, err error) {
	iter = Where(iter, pred)
	return Single(iter)
}
func Singlep(iter Enumerable, pred Predicate) (retval interface{}, err error) {
	iter = Where(iter, pred)
	return Single(iter)
}
// 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.