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


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end