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


DEBUG:Going to execute:
let idx := fresh "idx" in
           destruct id as [idx id]; repeat CoreTactics.destruct_sigma idx;
            simpl in id
DEBUG LTAC Evaluated term: idx
DEBUG LTAC Evaluated term: (sigma (λ _ : list A, list A))
Level 7: type of id
 has value 
(sigma (λ _ : list A, list A))