code: def crosslisting_feature(catalog, soup):
      
      listing = {}
      for elem in soup.coursedb.findAll('crosslisting'):
          seats = int(elem['seats'])
          crns = [safeInt(crn.string) for crn in elem.findAll('crn')]
  
          # we want to refer to the same object to save space
          cl = CrossListing(crns, seats)
          for crn in crns:
              listing[crn] = cl
      catalog.crosslistings = FrozenDict(listing)
  
      logger.info('Catalog has %d course crosslistings' % len(catalog.crosslistings))
docs: Parses all the crosslistings. These refer to the similar CRNs,
      such as a grad & undergrad level course.