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| = #|tl|)


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]