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


DEBUG:Going to execute:
CoreTactics.destruct_sigma idx