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:
fun => <coq-core.plugins.ltac::assumption@0>

File "./theories/All_Forall.v", line 1516, characters 84-94:
Warning: Notation map_length is deprecated since 8.20.
Use length_map instead.
[deprecated-syntactic-definition-since-8.20,deprecated-since-8.20,deprecated-syntactic-definition,deprecated,default]