string.index_next(S, 0, 4, 0x1f600)
string.index_next(S, 4, 5, 0xfffd)
string.index_next(S, 5, 6, 0xfffd)
string.index_next(S, 6, 10, 0x1f600)
string.index_next(S, 10, _, _) failed