Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → Type
  Q : B → C → Type
  R : A → C → Type
  l'' : list C
  a : All2 Q [] l''
  ============================
   (let H := CoreTactics.block in
    (∀ (x : A) (y : B) (z : C), P x y → Q y z → R x z) → All2 R [] l'')


DEBUG:Going to execute:
CoreTactics.block_goal
Level 4: evaluation returns
idtac of type tacvalue

Level 2: evaluation returns
idtac of type tacvalue

Level 1: evaluation returns
idtac of type tacvalue