Checking an Object's Class

  A variation on the technique described above is to check whether an object
  belongs to a particular class. This can be done as follows:

    IF obj:class == Window()

  This uses the class message to obtain a reference to the class object
  for the class to which obj belongs. By comparing this to the reference
  to the class object returned by a class function such as Window()
  (assuming Window is a class), we can determine whether obj belongs to
  that class or not.