Goal:
  
  A : Type
  P : nat → A → A → Type
  idx0 : nat
  idx1 : list A
  idx : list A
  a : OnOne2i P idx0 idx1 idx
  idx3 : nat
  idx2 : sigma (λ _ : list A, list A)
  b : OnOne2i P (pr1 {| pr1 := idx3; pr2 := idx2 |})
        (pr1 (pr2 {| pr1 := idx3; pr2 := idx2 |}))
        (pr2 (pr2 {| pr1 := idx3; pr2 := idx2 |}))
  H : NoConfusion_OnOne2i A P
        {|
          pr1 := {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |};
          pr2 := a
        |} {| pr1 := {| pr1 := idx3; pr2 := idx2 |}; pr2 := b |}
  ============================
   ({|
      pr1 := {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |};
      pr2 := a
    |} = {| pr1 := {| pr1 := idx3; pr2 := idx2 |}; pr2 := b |})


DEBUG:Going to execute:
CoreTactics.destruct_sigma idx