Goal:
  
  A : Type
  f : A -> bool
  g : A -> bool
  Hfg : f =1 g
  f' : A -> bool
  g' : A -> bool
  Hfg' : f' =1 g'
  i : A
  ============================
   (predI f f' i = predI g g' i)


DEBUG:Going to execute:
rewrite /predI /=