Goal:
  
  elt : Type
  gt_tree_dec : forall (x : T.t) (t : M.Raw.tree elt),
                {M.Raw.gt_tree x t} + {~ M.Raw.gt_tree x t}
  x : T.t
  t : M.Raw.tree elt
  l : M.Raw.tree elt
  k : M.Raw.key
  n : elt
  r : M.Raw.tree elt
  z : Int.Z_as_Int.t
  l0 : T.lt x k
  g : M.Raw.gt_tree x l
  pf : ~ M.Raw.gt_tree x r
  ============================
   (~ M.Raw.gt_tree x (M.Raw.Node l k n r z))


DEBUG:Going to execute:
intro f; apply pf; hnf in *