Goal:
  
  A : Type
  P : A → A → Type
  l' : list A
  f : A → A
  hp : ∀ x y : A, P (f x) y → ∑ y' : A, y = f y'
  o1' : list A
  o1 : list A
  o : OnOne2 P o1' o1
  ============================
   ({| pr1 := []; pr2 := l' |} = {| pr1 := o1'; pr2 := o1 |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      ∑ l'' : list A, OnOne2 (λ x y : A, P (f x) (f y)) [] l'')


DEBUG:Going to execute:
let ty := type of p in
           let eliminator := eval simpl in DepElim.elim in
           tac p eliminator