Goal:
  
  A : Type
  P : A → Prop
  Q : A → Prop
  a : A
  l : list A
  a0 : A
  l' : list A
  IHl : All P l → All Q l' → #|l| = #|l'| → All2 (λ x y : A, P x ∧ Q y) l l'
  p : P a
  ha : All P l
  hb1 : list A
  hb : All Q hb1
  ============================
   (a0 :: l' = hb1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      S #|l| = S #|l'| → All2 (λ x y : A, P x ∧ Q y) (a :: l) (a0 :: l'))


DEBUG:Going to execute:
fun p =>
             DepElim.introduce p;
              DepElim.elim_case p || destruct p || (case p; clear p)