let count = gen() {
    var n = 0
    loop {
        yield n
        n := n + 1
    }
}

let powers = gen(m) {
    for n in count() {
        yield n ** m
    }
}

let filtered = gen(s1, s2) {
    var v = s1()
    var f = s2()
    loop {
        if v > f {
            f := s2()
            continue
        } else if v < f {
            yield v
        }
        v := s1()
    }
}

let squares = powers(2)
let cubes = powers(3)
let f = filtered(squares, cubes)

// Drop first 20 items.
for i in range(20) {f()}
// Print next 10 items.
for i in range(10) {print(f())}