Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  x : A
  l : list A
  p : P x
  H1 : All P l
  H2 : ∀ (x : A) (y : B), In y [] → P x → Q x y
  Hl : 0 = S #|l|
  Hl0 := {| pr1 := S #|l|; pr2 := Hl |} : sigma (λ index : nat, 0 = index)
  ============================
   (let H := CoreTactics.block in
    (∀ l' : list B,
       (∀ (x : A) (y : B), In y l' → P x → Q x y)
       → #|l'| = #|l| → All2 Q l l')
    → let H0 := CoreTactics.block in All2 Q (x :: l) [])


DEBUG:Going to execute:
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