bug in unification

[Enrico Tassi]

$ elpi
goal> pi x\ (w\ f (y\ X x y) w) = (z\ X x z).

Success:
  X = c0 \ c1 \ f (c2 \ X0 c0 c2) c1

[Enrico Tassi]

goal> (w\ f (y\ X y) w) = (z\ X z).  % works
goal> f (y\ X y) = (z\ X z).  % fails (that is intended)

[Enrico Tassi]

pi w \ f (y\ X y) w = X w.  % works