Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  b : B
  l' : list B
  x : A
  l : list A
  p : P x
  H1 : All P l
  H2 : ∀ (x : A) (y : B), In y (b :: l') → P x → Q x y
  Hl1 : nat
  Hl : S #|l'| = Hl1
  ============================
   (S #|l| = Hl1
    → 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) (b :: 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' := Hl
of type
constr
id := Hl1
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' := Hl
of type
constr
id := Hl1
of type
constr of type tacvalue