old_contents: """Category Theory module.
  
  Provides some of the fundamental category-theory-related classes,
  including categories, morphisms, diagrams.  Functors are not
  implemented yet.
  """
  
  from baseclasses import Object, Morphism
  
new_contents: """
  Category Theory module.
  
  Provides some of the fundamental category-theory-related classes,
  including categories, morphisms, diagrams.  Functors are not
  implemented yet.
  
  The general reference work this module tries to follow is
  
    [JoyOfCats] J. Adamek, H. Herrlich. G. E. Strecker: Abstract and
                Concrete Categories. The Joy of Cats.
  
  The latest version of this book should be available for free download
  from
  
     katmat.math.uni-bremen.de/acc/acc.pdf
  
  """
  
  from baseclasses import Object, Morphism
  
subject: Update the comment in __init__.py to include the principal reference.