var my_list = [1, 2, 3];
print(3 in my_list); # expect: true
print(4 in my_list); # expect: false

class MyClass {

    __init__() {
        this.x = 1;
    }

    contains(value) {
        return value == "x";
    }

    other(value) {
        return value == "y";
    }
}

var my_object = MyClass();

my_object.other(3);


print("x" in my_object); # expect: true
print("z" in my_object); # expect: false

class BadClass {}

var bad_object = BadClass();
"x" in bad_object; # expect runtime error: Undefined property 'contains'.