Goal:
  
  A : Type
  B : Type
  C : Type
  P : nat → A → B → Type
  Q : B → C → Type
  R : nat → A → C → Type
  S : nat → A → C → Type
  n : nat
  c : All2i R n [] []
  H : ∀ (n : nat) (x : A) (y : B) (z : C),
        P n x y → Q y z → R n x z → S n x z
  ============================
   (let H := CoreTactics.block in All2i S n [] [])


DEBUG:Going to execute:
fun id' =>
             match id' with
             | id => idtac
             | _ => revert id'; CoreTactics.revert_until id
             end