Goal:
  
  A : Type
  P : A → A → Type
  Q : A → Type
  HPQ : ∀ x y : A, Q x → P x y → Q y
  hd : A
  hd' : A
  tl : list A
  p : P hd hd'
  H1 : list A
  H : All Q H1
  ============================
   (hd :: tl = H1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All Q (hd' :: tl))


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