c := s[i]
		if c < utf8.RuneSelf {
			i++
			invalid = false
			b = append(b, c)
			continue
		}
		_, wid := utf8.DecodeRune(s[i:])
		if wid == 1 {
			i++
			if !invalid {
				invalid = true
				b = append(b, replacement...)
			}
			continue
		}
		invalid = false
		b = append(b, s[i:i+wid]...)
		i += wid
	}
	return b
}