code: def num_mutations(self):
          '''
          :return: number of mutations in the container
          '''
          self._initialize()
          res = super(Container, self).num_mutations()
          return ref num_mutations(self):
          '''
          :return: number of mutations in the container
          '''
          self._initialize()
          res = super(Container, self).num_mutations()
          return res
docs: :return: number of mutations in the container