Goal:
  
  A : Type
  B : Type
  t : A
  P : B → A → A → Type
  b : B
  bs : list B
  hd' : A
  tl : list A
  p : P b t hd'
  e : #|bs| = #|tl|
  ============================
   (Some b = Some b × P b t hd')


DEBUG:Going to execute:
try arith_congr
Level 0: Backtrace:

No product even after head-reduction.
Level 0: Backtrace:

No product even after head-reduction.
Level 3: Backtrace:

No product even after head-reduction.
Level 2: Backtrace:

No product even after head-reduction.
Level 1: Backtrace:

No product even after head-reduction.
Level 0: Backtrace:

No product even after head-reduction.
Level 0: Backtrace:

No product even after head-reduction.