func (da *Cedar) PrefixMatch(key []byte, num int) (ids []int) {
	for from, i := 0, 0; i < len(key); i++ {
		to, err := da.Jump(key[i:i+1], from)
		if err != nil {
			break
		}
		if _, err := da.Value(to); err == nil {
			ids = append(ids, to)
			num--
			if num == 0 {
				return
			}
		}
		from = to
	}
	return
}
func (da *Cedar) PrefixMatch(key []byte, num int) (ids []int) {
	for from, i := 0, 0; i < len(key); i++ {
		to, err := da.Jump(key[i:i+1], from)
		if err != nil {
			break
		}
		if _, err := da.Value(to); err == nil {
			ids = append(ids, to)
			num--
			if num == 0 {
				return
			}
		}
		from = to
	}
	return
}
// PrefixMatch returns a list of at most `num` nodes which match the prefix of the key.
// If `num` is 0, it returns all matches.
// For example, if the following keys were inserted:
//	id	key
//	19	abc
//	23	ab
//	37	abcd
// then
//	PrefixMatch([]byte("abc"), 1) = [ 23 ]				// match ["ab"]
//	PrefixMatch([]byte("abcd"), 0) = [ 23, 19, 37]		// match ["ab", "abc", "abcd"]