Goal:
    A : Type  B : Type  R : nat → A → B → Type  n : nat  b : All2i R n [] []
  ============================
   (NoConfusionHom_All2i A B R n [] [] All2i_nil b
    → let H := CoreTactics.block in All2i_nil = b)


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