title: Why doesn't 'Q' unify in this PROLOG program
body: <p>I am writing a PROLOG program in which the variable of interest (Q) refuses to unify. I have gotten around this with a hacky solution (include a <code>write</code> statement). But there has to be a way to make this unify, but for the love of me, I am not able to figure it out.</p>
  
  <p>I'd really appreciate any help.
  Thanks in advance.</p>
  
  <p>Here is my code (I have annotated wherever I have excluded code for brevity)</p>
  
  <pre><code>:- use_module(library(bounds)).
  :- use_module(library(lists)).
  
  solve([17],Q,_,_,_):- write(Q). %this is the hacky workaround
  solve(L,Q,1,3,2) :- jump(L,Q,N,1,3,2,R), solve(N,R,S,D,M), member([S|[D|[M|[]]]],[[1, 3, 2], [1, 9, 4], [2, 10, 5] this list contains 76 items - all of which are lists of length 3. I have omitted them here for the sake of brevity]).
  % there are about 75 other definitions for solve, all of which are structured exactly the same. The only difference is that the numbers in the input parameters will be different in each definition
  
  jump(L,Q,N,S,D,M,R):- member(S,L), not(member(D,L)), member(M,L), delete(L,S,X), delete(X,M,Y), append(Y,[D],N), append(Q,[[S,D]],R).
  
  cross_sol(Q) :- solve([5,9,10,11,17,24],Q,S,D,M), member([S,D,M], [
  I have edited out this list here for the sake of brevity. It is the same list found in the definition of solve
  ]).
  </code></pre>
  
  <p>For some reason, Q does not unify. Please help!
  Thank you!</p>