Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  idx0 : list B
  idx : sigma (λ _ : list A, list A)
  a : OnOne2All P (pr1 {| pr1 := idx0; pr2 := idx |})
        (pr1 (pr2 {| pr1 := idx0; pr2 := idx |}))
        (pr2 (pr2 {| pr1 := idx0; pr2 := idx |}))
  ============================
   (NoConfusion_OnOne2All A B P
      {| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |}
      {| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |})


DEBUG:Going to execute:
CoreTactics.destruct_sigma idx