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
  hb1 : sigma (λ _ : list B, list D)
  hb : All2 R (pr1 hb1) (pr2 hb1)
  ============================
   ({| pr1 := b :: l; pr2 := l''' |} = hb1
    → ∀ (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:
hnf in id
Level 4: evaluation returns
hnf in id;
 match type of id with
 | sigma _ =>
     let xn := fresh id "'" in
     destruct id as [xn id]; DepElim.decompose_exists xn id;
      cbv beta iota delta [TODO projection TODO projection] in id, id';
      DepElim.decompose_exists id id'
 | _ => cbv beta iota delta [TODO projection TODO projection] in id, id'
 end
where
id' := hb
of type
constr
id := hb1
of type
constr of type tacvalue

Level 3: evaluation returns
hnf in id;
 match type of id with
 | sigma _ =>
     let xn := fresh id "'" in
     destruct id as [xn id]; DepElim.decompose_exists xn id;
      cbv beta iota delta [TODO projection TODO projection] in id, id';
      DepElim.decompose_exists id id'
 | _ => cbv beta iota delta [TODO projection TODO projection] in id, id'
 end
where
id' := hb
of type
constr
id := hb1
of type
constr of type tacvalue