code: func (s StringSlice) ExistsAllIgnoreCase(str []string) bool {
  	for _, v := range str {
  		if !s.ExistsIgnoreCase(v) {
  			return false
  		}
  	}
  
  	return true
  }
docs: //ExistsAllIgnoreCase determine whether all specified strings exists into
  // current slice (ignores letter casing).