macro_rules! fct {
    (@step $s:literal ()) => { 1 };
    (@step $s:literal ($one:literal)) => { 1 };
    (@step $s:literal ($drop:literal, $($smth:literal),*)) => {
        (fct!(@mult ($($smth),*) ($($smth),*)))
    };
    
    (@mult ($($e:literal),*) $rec:tt) => {
        $(
            fct!(@step $e $rec) +
        ) + fct!(@step 0 $rec)
    };
    ($($a:literal),*) => {
        fct!(@step 0 ($($a),*))
    };
}
#[test]
fn fct() {
    let n = fct!(1,1,1,1,1,1,1);
    assert_eq!(n, 5040);
}