import <Utilities/Sequence.sl>;

lcsBack(a(1), b(1)) :=
    let
        aSub := allButLast(a);
        bSub := allButLast(b);

        x := lcsBack(a, bSub);
        y := lcsBack(aSub, b);
    in
        [] when size(a) = 0 or size(b) = 0
    else
        lcsBack(aSub, bSub) ++ [last(a)] when last(a) = last(b)
    else
        x when size(x) > size(y)
    else
        y;

main(args(2)) :=
        lcsBack(args[1], args[2]) when size(args) >=2
    else
        lcsBack("thisisatest", "testing123testing");