// code point satisfying f(c), or -1 if none do.
func LastIndexFunc(s []byte, f func(r rune) bool) int {
	return lastIndexFunc(s, f, true)
}

// indexFunc is the same as IndexFunc except that if
// truth==false, the sense of the predicate function is
// inverted.
func indexFunc(s []byte, f func(r rune) bool, truth bool) int {
	start := 0
	for start < len(s) {
		wid := 1
		r := rune(s[start])
		if r >= utf8.RuneSelf {

+ The PR title is formatted as follows: `net/http: frob the quux before blarfing`
  + The package name goes before the colon
  + The part after the colon uses the verb tense + phrase that completes the blank in,
    "This change modifies Go to ___________"
  + Lowercase verb after the colon
  + No trailing period
  + Keep the title as short as possible. ideally under 76 characters or shorter

		}
	}

	n = len(b0) - len(b)
	switch {
	case err == io.EOF:
		return n, errMissData // Less data in dense file than sparse file
	case err != nil:
		return n, err
	case sr.logicalRemaining() == 0 && sr.physicalRemaining() > 0:
		return n, errUnrefData // More data in dense file than sparse file
	case finished:
		return n, io.EOF
	default:
		return n, nil
	}
}