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
  hd' : A
  tl : list A
  p : P b hd hd'
  e : #|bs| = #|tl|
  q : Q hd
  H : All Q tl
  ============================
   (Q ?x)


DEBUG:Going to execute:
solve [ inversion H ]
Level 3: Backtrace:

The type of hd' is not inductive.
Level 3: Backtrace:

No applicable tactic.