Broken eta-expansion in commit 6e859ec

[Nathan Guermond]

The following worked in #107

mode (foo i i).
(foo X (x1 \ (x2 \ X x2))) :- (print X).
%% Fails, but should output `y`
main :- (foo y (x1 \ (x2 \ y x2))). 

but seems to break in commit 6e859ec.

[Enrico Tassi]

Thanks for bisecting