Goal:
  
  A : Type
  P : A → A → Type
  a : A
  l : list A
  f : A → A
  hd' : A
  hp : ∀ x y : A, P (f x) y → ∑ y' : A, y = f y'
  IHl : ∀ l' : list A,
          OnOne2 P (map f l) l'
          → ∑ l'' : list A, OnOne2 (λ x y : A, P (f x) (f y)) l l''
  p : P (f a) hd'
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    ∑ l'' : list A, OnOne2 (λ x y : A, P (f x) (f y)) (a :: l) l'')


DEBUG:Going to execute:
match T with
           | context [ CoreTactics.block ] =>
               change T; CoreTactics.intros_until_block
           end