code: func appendStrNotExist(str string, arr []string) []string {
  	// check if a `str` exist in `arr`
  	isStrInArr := func(str string, arr []string) bool {
  		for _, s := range arr {
  			if str == s {
  				return true
  			}
  		}
  		return false
  	}
  
  	if !isStrInArr(str, arr) {
  		arr = append(arr, str)
  	}
  	return arr
  }
docs: // append `str` to `arr` if `str` not exist in `arr`