code: func checkKey(s string) {
  	for _, char := range s {
  		if (char < 'A' || char > 'Z') && char != '_' {
  			panic(errKey)
  		}
  	}
  }
docs: // == Unset variables
  //
  // _Unset unsets the key in the given filename.
  /*func _Unset(filename, key string) error {
  
  }*/
  // == Utility
  //
  // It is a common practice to name all environment variables with only English
  // capital letters and underscore (_) signs.
  // checkKey reports whether the key uses English capital letters and underscore
  // signs.