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


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
((All Q l → All (λ x : A, P x × Q x) l)
 → let H := CoreTactics.block in All (λ x : A, P x × Q x) (x :: l))