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


DEBUG:Going to execute:
trivial
Level 1: Backtrace:

Unable to find an instance for the variable x.
Level 0: Backtrace:

Unable to find an instance for the variable x.