let filter = fn (arr, f) {
    let iter = fn (arr, accumulated) {
        if (len(arr) == 0) {
            accumulated
        } else {
            let head = first(arr);
            let tail = rest(arr);
            if (f(head)) {
                iter(tail, push(accumulated, head))
            } else {
                iter(tail, accumulated)
            }
        }
    };
    iter(arr, [])
};
let a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 100];
let is_even = fn (x) {
    x % 2 == 0
};
filter(a, is_even);