Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  H2 : ∀ (x : A) (y : B), In y [] → P x → Q x y
  Hl1 : nat
  Hl : 0 = Hl1
  ============================
   (0 = Hl1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All2 Q [] [])


DEBUG:Going to execute:
fun p =>
             DepElim.introduce p;
              DepElim.elim_case p || destruct p || (case p; clear p)