Goal:
  
  A : Type
  P : A → A → Type
  a : sigma
        (λ index : sigma (λ _ : list A, list A),
           OnOne2 P (pr1 index) (pr2 index))
  b : sigma
        (λ index : sigma (λ _ : list A, list A),
           OnOne2 P (pr1 index) (pr2 index))
  H : a = b
  ============================
   (NoConfusion_OnOne2 A P a b)


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end