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This code correctly calculates the number of elements in a difference list when I run it on GNU Prolog and SWI Prolog. However, Yap enters an infinite loop.
count(X-X1,0) :- unify_with_occurs_check(X,X1), !.
count([H|T]-T1,N) :- count(T-T1,M), N is M+1.
?- count([1,2|A]-A,N).
Why would a Prolog interpreter (like Yap) not terminate when asked this query?
There seems to be a bug in Yap 5.1.3
Newer versions (tested with Yap 6.2.2) work fine: