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


DEBUG:Going to execute:
CoreTactics.destruct_sigma idx
Level 5: evaluation returns
idtac of type tacvalue