Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → C → Type
  g : A → B → C
  x : A
  xs : list A
  ih : ∀ l' : list B,
         #|xs| = #|l'|
         → All2 R xs (map2 g xs l')
           → All2 (λ (x : A) (y : B), R x (g x y)) xs l'
  y : B
  ys : list B
  eq : #|xs| = #|ys|
  z : All2 R (x :: xs) (g x y :: map2 g xs ys)
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2 (λ (x : A) (y : B), R x (g x y)) (x :: xs) (y :: ys))


DEBUG:Going to execute:
fun id cont =>
             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