Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  R' : ∀ (a : A) (b : B), R a b → Type
  b : All2_dep R' All2_nil
  H : NoConfusionHom_All2_dep A B R R' [] [] All2_nil All2_dep_nil b
  ============================
   (let H := CoreTactics.block in All2_dep_nil = b)


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