chars([a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z]).

concats([], [], []).
concats([X | Xs], [Y | Ys], [Z | Zs]) :-
    atom_concat(X, Y, Z),
    concats(Xs, Ys, Zs).

doconcat(0, _, SoFar, SoFar).
doconcat(N, Cur, SoFar, Res) :-
    N > 0,
    N1 is N - 1,
    chars(C),
    concats(C, Cur, New),
    append(SoFar, New, SoFar1),
    doconcat(N1, New, SoFar1, Res).


vars(0, []).
vars(N, [V | L]) :-
    N > 0,
    chars(C),
    random_member(V, C),
    N1 is N - 1,
    vars(N1, L).

nums(0, []).
nums(N, [X | L]) :-
    N > 0,
    random_between(1, 10, X),
    N1 is N - 1,
    nums(N1, L).

allx(0, []).
allx(N, [x | L]) :- N > 0, N1 is N - 1, allx(N1, L).

test2(N, L, M, R, Len, RLen) :- chars(C), doconcat(N, C, [], L), length(L, Len), nums(Len, M), solvelists(L, M, R), length(R, RLen).