Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  idx0 : list A
  idx : list A
  a : All2_fold P idx0 idx
  idx2 : list A
  idx1 : list A
  b : All2_fold P (pr1 {| pr1 := idx2; pr2 := idx1 |})
        (pr2 {| pr1 := idx2; pr2 := idx1 |})
  H : NoConfusion_All2_fold A P
        {| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |}
        {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |}
  ============================
   ({| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |} =
    {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |})


DEBUG:Going to execute:
CoreTactics.destruct_sigma idx