interface Maybe {}
class Just implements Maybe {
    private $a;

    public function __construct($a) {
        $this->a = $a;
 
    }
    public function patternMatched(callable $fn) {
        return $fn($this->a);
    }
}
class Nothing implements Maybe {
    
    public function patternMatched(callable $fn) {
        return $fn();
    }
}