code: def _compile_references(self, url, tree):
          '''
          Returns a list of catalog reference URLs for the current catalog
          :param str url: URL for the current catalog
          :param lxml.etree.Eleemnt tree: Current XML Tree
          '''
          references = []
          for ref in tree.findall('.//{%s}catalogRef' % INV_NS):
              # Check skips
              title = ref.get("{%s}title" % XLINK_NS)
              if any([x.match(title) for x in self.skip]):
                  logger.info("Skipping catalogRef based on 'skips'.  Title: %s" % title)
                  continue
              references.append(construct_url(url, ref.get("{%s}href" % XLINK_NS)))
          return referencef _compile_references(self, url, tree):
          '''
          Returns a list of catalog reference URLs for the current catalog
          :param str url: URL for the current catalog
          :param lxml.etree.Eleemnt tree: Current XML Tree
          '''
          references = []
          for ref in tree.findall('.//{%s}catalogRef' % INV_NS):
              # Check skips
              title = ref.get("{%s}title" % XLINK_NS)
              if any([x.match(title) for x in self.skip]):
                  logger.info("Skipping catalogRef based on 'skips'.  Title: %s" % title)
                  continue
              references.append(construct_url(url, ref.get("{%s}href" % XLINK_NS)))
          return references
docs: Returns a list of catalog reference URLs for the current catalog
          :param str url: URL for the current catalog
          :param lxml.etree.Eleemnt tree: Current XML Tree