Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  Q : B → A → Type
  b : B
  bs : list B
  hd : A
  tl : list A
  hd' : A
  p : P b hd hd'
  e : #|bs| = #|tl|
  X : Q b hd
  X0 : All2 Q bs tl
  ============================
   (#|bs| = #|tl|)


DEBUG:Going to execute:
F x2 x1 x0
Level 0: Backtrace:

P is used in hypothesis p.
Level 0: Backtrace:

P is used in hypothesis p.
Level 0: Backtrace:

P is used in hypothesis p.
Level 0: Backtrace:

P is used in hypothesis p.
Level 0: Backtrace:

P is used in hypothesis p.
Level 1: Backtrace:

P is used in hypothesis p.
Level 0: Backtrace:

P is used in hypothesis p.