dif/2 weak with occurs_check

[UWN]

?- set_prolog_flag(occurs_check, true).
   true.
?- dif(-X,X).
   true. % expected
?- dif(Y,X),Y= -X.
   Y = - X, dif:dif(- X,X). % unexpected ; no dif expected
?- dif(Y,X),Y= -X, X\= Y.
   Y = - X, dif:dif(- X,X). % they are no longer unifiable, and thus the dif is redundant