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


DEBUG:Going to execute:
rewrite map_length
Level 1: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.