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


DEBUG:Going to execute:
CoreTactics.destruct_sigma idx
Level 6: evaluation returns
idtac of type tacvalue