// fine. It could also shrink but that falls out naturally.
	b := make([]byte, 0, len(s))
	for i := 0; i < len(s); {
		r, wid := utf8.DecodeRune(s[i:])
		r = mapping(r)
		if r >= 0 {
			b = utf8.AppendRune(b, r)
		}
		i += wid
	}
	return b
}

// Despite being an exported symbol,
// Repeat is linknamed by widely used packages.
// Notable members of the hall of shame include:

		m := utf8.RuneLen(needle)
		for o := m; o > 0; {
			_, sz := utf8.DecodeLastRune(buf)
			copy(buf[len(buf)-sz:], "\x00\x00\x00\x00")
			buf = buf[:len(buf)-sz]
			o -= sz
		}
		buf = utf8.AppendRune(buf[:n-m], needle)

		n -= m // adjust for rune len
		for i := 0; i < b.N; i++ {
			j := IndexRune(buf, needle)
			if j != n {
				b.Fatal("bad index", j)
			}
		}
		for i := range buf {