slices.go
func (s StringSlice) IndexOf(str string) int {
	for i, v := range s {
		if str == v {
			return i
		}
	}

	return -1
}
func (s StringSlice) IndexOf(str string) int {
	for i, v := range s {
		if str == v {
			return i
		}
	}

	return -1
}
// IndexOf looks for specified string into current slice.