Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  idx2 : list A
  idx1 : list A
  b : All2_fold P idx2 idx1
  H : NoConfusion_All2_fold A P
        {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := All2_fold_nil P |}
        {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |}
  ============================
   (let H := CoreTactics.block in
    {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := All2_fold_nil P |} =
    {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |})


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