Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  idx2 : list A
  idx1 : list A
  b : All2_fold P idx2 idx1
  ============================
   (∀ e : 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
      noConfusion_All2_fold_obligation_2 A P
        {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := All2_fold_nil P |}
        {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |}
        (noConfusion_All2_fold_obligation_1 A P
           {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := All2_fold_nil P |}
           {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |} e) = e)


DEBUG:Going to execute:
match goal with
           | |- ?T => change (let _ := CoreTactics.block in T)
           end