new_contents: class a(object):
      def __init__(self):
          self.v=0
      def m(self):
          print 'a.m'
      def x(self):
          print 'a.x'
  class b(a):
      def n(self):
          print 'b.n'
  class c(b):
      def m(self):
          print 'c.m'
      def n(self):
          print 'c.n'
        
  
subject: test classes