dif/2 incorrect Ⅳ

[UWN]

By adding an unrelated equation sharing a variable, success turns into failure.

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?- set_prolog_flag(occurs_check,true).
true.

?- -X=X.
false.

?- dif(A, B),         A=[D|C], B=[E|D], A=[E|B], E = e.
A = [e, e|e],
B = C, C = [e|e],
D = E, E = e.

?- dif(A, B),  _=C,       A=[D|C], B=[E|D], A=[E|B], E = e.
false, unexpected.

[JanWielemaker]

Interesting. Possibly variable ordering plays a role (of course this should not be the case).

[UWN]

Maybe more helpful:

?- dif(A, B),       A=[D|C], B=[E|D], A=[F|B],E=F, E = e.
A = [e, e|e],
B = C, C = [e|e],
D = E, E = F, F = e.

?- dif(A, B), _=C,  A=[D|C], B=[E|D], A=[F|B],E=F, E = e.
false, unexpected.

?- dif(A, B), _=C,  A=[D|C], B=[E|D], A=[F|B],E=F.
false, unexpected.

?- dif(A, B), _=C,  A=[D|C], B=[E|D], A=[F|B].
A = [F, E|F],
B = C, C = [E|F],
D = F.