sealed interface Shape {}
    record Square(int x) implements Shape {}
    record Rectangle(int l, int w) implements Shape {}
    record Circle(int r) implements Shape {}
    
    double getArea(Shape s) {
        // Exhaustively checks for all alternatives.
        return switch (s) {
            case Square(var x) -> x * x;
            case Rectangle(var l, var w) -> l * w;
            case Circle(var r) -> Math.PI * r * r;
        }
    }