Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  b : B
  l' : list B
  H2 : ∀ (x : A) (y : B), In y (b :: l') → P x → Q x y
  Hl1 : nat
  Hl : S #|l'| = Hl1
  ============================
   (0 = Hl1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All2 Q [] (b :: l'))


DEBUG:Going to execute:
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