Goal:
  
  A : Type
  B : Type
  C : Type
  R : nat → A → C → Type
  g : A → B → C
  x : A
  xs : list A
  ih : ∀ (l' : list B) (k : nat),
         All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) k xs l'
         → All2i R k xs (map2 g xs l')
  y : B
  ys : list B
  k : nat
  z : All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) k 
        (x :: xs) (y :: ys)
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in R k x (g x y))


DEBUG:Going to execute:
let id' := fresh id in
           get_signature_pack id id'; hnf in (value of id');
            hnf in (type of id');
            lazymatch goal with
            | id':=?v:_
              |- context [ id ] =>
                  generalize (eq_refl : v = id'); clearbody id'; simpl in id';
                   cont id id' id v
            | id':=?v:_
              |- _ =>
                  let id'1 := fresh id' in
                  let id'2 := fresh id' in
                  set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
                   hnf in (value of id'1), (value of id'2);
                   try red in (type of id'2);
                   match goal with
                   | id'1:=?t:_
                     |- _ =>
                         generalize (eq_refl : t = id'1); clearbody id'2 id'1;
                          clear id' id; try unfold signature in id'2;
                          hnf in id'2; simpl in id'2; rename id'2 into id;
                          cont id id id'1 t
                   end
            end