code: def xml(self, attribs = None,elements = None, skipchildren = False):
          
          if self.set is False or self.set is None:
              if len(self.data) == 0: #just skip if there are no children
                  return None
              else:
                  raise ValueError("No set specified or derivable for annotation layer " + self.__class__.__name__)
          return super(AbstractAnnotationLayer, self).xml(attribs, elements, skipchildren)
docs: See :meth:`AbstractElement.xml`