I replaced your different_lists and list_sequence with pure constraint versions (instead of your nondeterministic versions), and the changed program runs in practically the same amount of time. Thus, even if I don't understand it, your custom labeling algorithm works well.

The new definitions are as follow. The vdiffer predicate is defined in the prolog code elsewhere in this thread, whereas lex_chain is a new library predicate in Sicstus 4. 

different_lists(X, Y) :- vdiffer(X, Y).
list_sequence(X) :- lex_chain(X, [op(#<)]).

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If, however, you replace only list_sequence but keep your different_lists proceduce, the program gets somewhat faster (shows how the optimized library that comes with Sicstus is better than the proceduces I cobble together).

In reply to Re^5: Seven by seven farming puzzle by ambrus
in thread Seven by seven farming puzzle by ambrus

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