code: func substitute(src, sub, repl string) string {
  	idx := strings.Index(src, sub)
  	for idx >= 0 {
  		src = src[:idx] + repl + src[idx+len(sub):]
  		idx = strings.Index(src, sub)
  	}
  	return src
  }
docs: // substitute replaces the substring sub with the string repl.