Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  R : C → D → Type
  f : A → C
  g : B → D
  a : A
  l : list A
  IHl : ∀ l' : list B,
          All2 R (map f l) (map g l')
          → All2 (λ (x : A) (y : B), R (f x) (g y)) l l'
  H1' : list C
  H1 : list D
  H : All2 R H1' H1
  ============================
   ({| pr1 := f a :: map f l; pr2 := [] |} = {| pr1 := H1'; pr2 := H1 |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      All2 (λ (x : A) (y : B), R (f x) (g y)) (a :: l) [])


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