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


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

No such assumption.
Level 0: Backtrace:

No such assumption.