module Standard {
    module Algorithm {
        Flatten() {
            return function(range) {
                return function()[range := std.move(range), first := true, begin := decltype(range())->decay()] {
                    if (first) {
                        begin = range();
                        first = false;
                    }
                    if (~begin) { return decltype((*begin)())->decay(Containers.None); }
                    var := (*begin)();
                    while(~var) {
                        begin = range();
                        if (~begin) return Containers.None;
                        var = (*begin)();
                    }
                    return std.move(var);
                };
            };
        }
    }
}