func checkKey(s string) {
	for _, char := range s {
		if (char < 'A' || char > 'Z') && char != '_' {
			panic(errKey)
		}
	}
}
func checkKey(s string) {
	for _, char := range s {
		if (char < 'A' || char > 'Z') && char != '_' {
			panic(errKey)
		}
	}
}
// == 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.