code: func commatize(sep, s string) string {
  	// If no dot, don't do anything.
  	if !strings.ContainsRune(s, '.') {
  		return s
  	}
  	var b bytes.Buffer
  	fs := strings.SplitN(s, ".", 2)
  
  	l := len(fs[0])
  	for i := range fs[0] {
  		b.Write([]byte{s[i]})
  		if i < l-1 && (l-i)%3 == 1 {
  			b.WriteString(sep)
  		}
  	}
  
  	if len(fs) > 1 && len(fs[1]) > 0 {
  		b.WriteString(".")
  		b.WriteString(fs[1])
  	}
  
  	return b.String()
  }
docs: // commatize returns a number with sep as thousands separators. Handles
  // integers and plain floats.