Goal:
    A : Type  P : tree A -> Type
  ============================
   (P Empty ->
    (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)) -> 
    tree' A -> True)


DEBUG:Going to execute:
let ty := type of B in
           intro || (let H := fresh in
                     intro
                     H)
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.