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'
  ha1 : list A
  ha : All P ha1
  ============================
   (a :: l = ha1
    → let H := CoreTactics.block in
      All Q (a0 :: l')
      → 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 tac p =>
             let ty := type of p in
             let eliminator := eval simpl in DepElim.elim in
             tac p eliminator