Goal:
  
  A : Type
  P : A → A → Type
  idx : sigma (λ _ : list A, list A)
  a : OnOne2 P (pr1 idx) (pr2 idx)
  b : sigma
        (λ index : sigma (λ _ : list A, list A),
           OnOne2 P (pr1 index) (pr2 index))
  e : NoConfusion_OnOne2 A P {| pr1 := idx; pr2 := a |} b
  ============================
   (noConfusion_OnOne2_obligation_2 A P {| pr1 := idx; pr2 := a |} b
      (noConfusion_OnOne2_obligation_1 A P {| pr1 := idx; pr2 := a |} b e) =
    e)


DEBUG:Going to execute:
CoreTactics.destruct_sigma idx