Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  Q : A → Type
  HPQ : ∀ (i : B) (x y : A), Q x → P i x y → Q y
  b : B
  bs : list B
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2All P bs tl tl'
  IHX : All Q tl → All Q tl'
  q : Q hd
  H : All Q tl
  ============================
   (let H := CoreTactics.block in All Q (hd :: tl'))


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in _ => intros _
           | |- _ => try (intro; CoreTactics.intros_until_block)
           end