Goal:
  
  A : Type
  B : Type
  P : A → B → B → Type
  Q : A → B → B → Type
  H1 : ∀ (x : A) (y z : B), P x y z → Q x y z
  H2 : ∀ (x : A) (y : B), Q x y y
  b : A
  a : A
  bs : list A
  hd : B
  hd' : B
  p : P b hd hd'
  e : #|a :: bs| = #|[]|
  ============================
   (All3 Q (a :: bs) [] [])


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.