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)
  l : tree' A
  ============================
   True


DEBUG:Going to execute:
unify X Y; clear H' || clear H
Level 4: H
 has value 
P