Goal:
  
  A : Type
  cmp : A -> A -> comparison
  Hc : forall (c : comparison) (x y z : A),
       cmp x y = c -> cmp y z = c -> cmp x z = c
  He : forall x y : A, cmp x y = Eq -> x = y
  x : A
  y : A
  z : A
  q : comparison
  q' : comparison
  q'' : comparison
  Hqs : forall c : comparison, q = c -> q' = c -> q'' = c
  e : cmp x y = Gt
  e' : y = z
  ============================
   (q' = Gt -> (cmp x z ?? q'') = Gt)


DEBUG:Going to execute:
<coq-core.plugins.ltac::subst@1>