iterator.go
func (it *Iterator) ValidForPrefix(prefix []byte) bool {
	return it.Valid() && bytes.HasPrefix(it.item.key, prefix)
}
func (it *Iterator) ValidForPrefix(prefix []byte) bool {
	return it.Valid() && bytes.HasPrefix(it.item.key, prefix)
}
// ValidForPrefix returns false when iteration is done
// or when the current key is not prefixed by the specified prefix.