old_contents: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  
  
  class Graph(object):
      """Graph class."""
      pass
  
  
  def main():
      pass
  
  
  if __name__ == '__main__':
      main()
  
new_contents: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  
  
  class Vertex(object):
  	"""Vertex class.
  
      It uses a dict to keep track of the vertices which it's connected.
  	"""
  
  
  class Graph(object):
      """Graph class.
  
      It contains a dict to map vertex name to vertex objects.
      """
      pass
  
  
  def main():
      pass
  
  
  if __name__ == '__main__':
      main()
  
subject: Add Vertex class and doc strings for 2 classes