rejection sampling conditioner in prolog

cond.pl

% Two dices are thrown and the results added up.
% We sample when the sum is conditioned on the first dice landing on 2.

output(S) :-
  randvar(a, dice(X), X),
  dice(Y),
  S is X + Y.

dice(X) :-
  X is random(6) + 1.

:- dynamic randvar/3.
randvar(_, Goal, _) :-
  call(Goal).

sample_cond(N, Xs) :-
  asserta((
    randvar(a, Goal, X) :-
      !,
      call(Goal),
      X = 2)),
  sample_subcond(N, Xs).

sample_subcond(0, []).

sample_subcond(N, [X | Xs]) :-
  repeat,
  output(X),
  K is N - 1,
  sample_subcond(K, Xs).