Goal:
  
  X : Type
  Y : Type
  R : nat → X → Y → Type
  n : nat
  a0 : X
  xs : list X
  xs' : list X
  l : list Y
  a : All2i R n (a0 :: xs ++ xs') l
  ============================
   (let H := CoreTactics.block in
    (∀ (n : nat) (xs' : list X) (l : list Y),
       All2i R n (xs ++ xs') l
       → ∑ ys ys' : list Y,
           l = ys ++ ys' × All2i R n xs ys × All2i R (n + #|xs|) xs' ys')
    → let H0 := CoreTactics.block in
      ∑ ys ys' : list Y,
        l = ys ++ ys'
        × All2i R n (a0 :: xs) ys × All2i R (n + #|a0 :: xs|) xs' 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