old_contents: class AbstractVirtualHelixItem(object):
      """
      AbstractVirtualHelixItem is a base class for virtualhelixitem in all views.
      It includes slots that get connected in VirtualHelixItemController which
      can be overridden.
  
      Slots that must be overridden should raise an exception.
      """
      def virtualHelixNumberChangedSlot(self, virtual_helix, number):
          raise NotImplementedError
      def virtualHelixPropertyChangedSlot(self, virtual_helix, transform):
          pass
      def virtualHelixRemovedSlot(self, virtual_helix):
          raise NotImplementedError
      def strandAddedSlot(self, sender, strand):
          pass
  # end class
  
  
  
new_contents: class AbstractVirtualHelixItem(object):
      """
      AbstractVirtualHelixItem is a base class for virtualhelixitem in all views.
      It includes slots that get connected in VirtualHelixItemController which
      can be overridden.
  
      Slots that must be overridden should raise an exception.
      """
      def virtualHelixNumberChangedSlot(self, virtual_helix, number):
          pass
      def virtualHelixPropertyChangedSlot(self, virtual_helix, transform):
          pass
      def virtualHelixRemovedSlot(self, virtual_helix):
          pass
      def strandAddedSlot(self, sender, strand):
          pass
  # end class
  
  
  
subject: remove NotImplementedError exceptions from AbstractVirtualHelixItem