If I have (with clpfd loaded):
test1(Ps):-
permutation(Ps,[(a,1,1),(b,2,2),(c,3,1),(d,4,2)]),
Ps =[(L1,W1,X1),(L2,W2,X2),(L3,W3,Y1),(L4,W4,Y2)],
chain([W1,W2,W3,W4],#<).
And query:
?-test1(P).
P = [(a,1,1), (b,2,2), (c,3,1), (d,4,2)]
false
Which gives one answer as I expect, but with:
test2(Ps):-
permutation(Ps,[(a,1,1),(b,2,2),(c,3,1),(d,4,2)]),
Ps =[(L1,W1,X1),(L2,W2,X2),(L3,W3,Y1),(L4,W4,Y2)],
chain([X1,X2],#<).
Query:
?-test2(P):-
P = [(a,1,1), (b,2,2), (c,3,1), (d,4,2)]
P = [(a,1,1), (b,2,2), (d,4,2), (c,3,1)]
P = [(a,1,1), (d,4,2), (b,2,2), (c,3,1)]
P = [(a,1,1), (d,4,2), (c,3,1), (b,2,2)]
P = [(c,3,1), (b,2,2), (a,1,1), (d,4,2)]
P = [(c,3,1), (d,4,2), (a,1,1), (b,2,2)]
P = [(c,3,1), (b,2,2), (d,4,2), (a,1,1)]
P = [(c,3,1), (d,4,2), (b,2,2), (a,1,1)]
false
I do not expect answers like:
P = [(c,3,1), (b,2,2), (d,4,2), (a,1,1)]
P = [(c,3,1), (d,4,2), (b,2,2), (a,1,1)]
Where the terms with as are after the terms with bs. What am I misunderstanding?
I can now see what I was doing wrong.
As the answers:
P = [(c,3,1), (b,2,2), (d,4,2), (a,1,1)]
P = [(c,3,1), (d,4,2), (b,2,2), (a,1,1)]
Clearly have 1 and 2 in the third argument of the first two elements and so are meeting the chain constraint.