Goal:
  
  A : Type
  P : A → Type
  Q : A → Type
  x : A
  l : list A
  p : P x
  Hl : All P l
  Hl' : All Q (x :: l)
  ============================
   (let H := CoreTactics.block in
    (All Q l → All (λ x : A, P x × Q x) l)
    → let H0 := CoreTactics.block in All (λ x : A, P x × Q x) (x :: l))


DEBUG:Going to execute:
fun id cont =>
             DepElim.generalize_sig_gen id
              ltac:(fun id id' id'1 t =>
                      try rename id into id',id' into id; cont id'1 id)