// 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: