Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ : list A
  a : A
  Γ' : list A
  Hrel : All2_fold P Γ (a :: Γ')
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    ∑ d : A,
      nth_error Γ 0 = Some d
      × (let Γs := skipn 1 Γ in
         let Γs' := skipn 1 (a :: Γ') in All2_fold P Γs Γs' × P Γs Γs' d a))


DEBUG:Going to execute:
get_signature_pack id id'; hnf in (value of id');
            hnf in (type of id')
Level 3: evaluation returns
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
where
id' := Hrel0
of type
intropattern
id := Hrel
of type
constr
cont := fun id id' id'1 t => try rename id into id',id' into id; cont id'1 id
        where
        id := Hrel
        of type
        constr
        cont := fun id id' => DepElim.decompose_exists id id'
                where
                id := Hrel
                of type
                constr
        of type
        tacvalue
of type
tacvalue of type tacvalue

Level 2: evaluation returns
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
where
id' := Hrel0
of type
intropattern
id := Hrel
of type
constr
cont := fun id id' id'1 t => try rename id into id',id' into id; cont id'1 id
        where
        id := Hrel
        of type
        constr
        cont := fun id id' => DepElim.decompose_exists id id'
                where
                id := Hrel
                of type
                constr
        of type
        tacvalue
of type
tacvalue of type tacvalue

Level 1: evaluation returns
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
where
id' := Hrel0
of type
intropattern
id := Hrel
of type
constr
cont := fun id id' id'1 t => try rename id into id',id' into id; cont id'1 id
        where
        id := Hrel
        of type
        constr
        cont := fun id id' => DepElim.decompose_exists id id'
                where
                id := Hrel
                of type
                constr
        of type
        tacvalue
of type
tacvalue of type tacvalue