Goal:
  
  A : Type
  P : A → A → Type
  Q : A → Type
  HPQ : ∀ x y : A, Q x → P x y → Q y
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2 P tl tl'
  IHX : All Q tl → All Q tl'
  q : Q hd
  H : All Q tl
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All Q (hd :: tl'))


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in All Q (hd :: tl'))