function like Mathematica's Tuples

Thank you all for your answers. It was very helpful.
And here is my simple procedural aproach:

Tuples(S, N):=
block(
    K:length(S),
    counter:create_list(1, i, 1, N),
    res:[create_list(S[counter[i]],i,1,N)],
    for i:1 thru K^N-1 do (
        counter[1]:counter[1]+1,
        for j:1 thru N-1 do (
            if counter[j]>K then (
                counter[j]:1,
                counter[j+1]:counter[j+1]+1
            )
            else(
                return
            )
        ),
        res:append(res, [create_list(S[counter[i]],i,1,N)])
    ),
    res
)$

usage:
Tuples([a,b], 2);
[[a,a],[b,a],[a,b],[b,b]]

Tuples([[a,b],c], 2);
[[[a,b],[a,b]],[c,[a,b]],[[a,b],c],[c,c]]

Greetings
Piotr Slomski