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


DEBUG:Going to execute:
fun id =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')