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
  r : tree' A
  Heq_m : Node101 l r = m
  ============================
   True


DEBUG:Going to execute:
subst Y
Level 1: Backtrace:

Ltac variable X is bound to Node101 l r of type constr which cannot be
coerced to a variable.