Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  E : Type
  P : E → A → Type
  Q : C → A → Type
  R : B → D → Type
  f : B → C → E
  b : B
  l : list B
  l''' : list D
  hb : All2 R (b :: l) l'''
  hb0 := {| pr1 := {| pr1 := b :: l; pr2 := l''' |}; pr2 := hb |}
      : sigma
          (λ index : sigma (λ _ : list B, list D),
             All2 R (pr1 index) (pr2 index))
  ============================
   (∀ (H := CoreTactics.block) (x : C) (y : A) (l0 : list C) (l' : list A),
      Q x y
      → All2 Q l0 l'
        → #|l| = #|l0|
          → (∀ (x0 : B) (y0 : C) (z : A) (w : D),
               R x0 w → Q y0 z → P (f x0 y0) z)
            → (∀ (l : list B) (l''' : list D),
                 All2 R l l''' → #|l| = #|l0| → All2 P (map2 f l l0) l')
              → let H0 := CoreTactics.block in
                All2 P (map2 f (b :: l) (x :: l0)) (y :: 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