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)
  r : tree' A
  ============================
   True


DEBUG:Going to execute:
match goal with
           | H:?X
             |- _ =>
                 match goal with
                 | H':?Y
                   |- _ =>
                       match H with
                       | H' => fail 2
                       | _ => unify X Y; clear H' || clear H
                       end
                 end
           end