func make_counter(start: Int, step: Int): Func {
    let value = start
    ret func(): Int {
        value = value + step
        ret value
    }
}
let next = make_counter(0, 1)
let tens = make_counter(0, 10)
print("next: {next()}")
print("next: {next()}")
print("tens: {tens()}")
print("next: {next()}")
print("tens: {tens()}")