out = {

    decr = { out = add < (in; -1;); };

    n = tupEl < (in; 0;);
    a = tupEl < (in; 1;);
    b = tupEl < (in; 2;);

    out = if < (
        isZero < n;
        a;
        recur < (
            decr < n;
            b;
            add < (a;b;);
        );
    );

} < (in; 0; 1;);