Goal:
  
  A : Type
  P : tree A -> Type
  empty : P Empty
  node : forall l : tree A,
         P l ->
         forall (o : option A) (r : tree A),
         P r -> not_trivially_empty l o r -> P (Node l o r)
  m : tree' A
  l : tree' A
  x : A
  r : tree' A
  Heq_m : Node111 l x r = m
  ============================
   True


DEBUG:Going to execute:
subst X || subst Y