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
  ============================
   (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:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(All Q (a0 :: l')
 → let H := CoreTactics.block in
   S #|l| = S #|l'| → All2 (λ x y : A, P x ∧ Q y) (a :: l) (a0 :: l'))