Goal:
  
  A : Type
  P : A → Prop
  Q : A → Prop
  p : A → bool
  x : A
  l : list A
  H : Q x
  HQ : Forall Q l
  Hp : ∀ x : A, Q x → reflect (P x) (p x)
  IHHQ : Forall P l → forallb p l = true
  HP1 : list A
  HP : Forall P HP1
  ============================
   (x :: l = HP1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in forallb p (x :: l) = true)


DEBUG:Going to execute:
DepElim.decompose_exists id id'