code: def getMd5Checksum(self):
          
          references = sorted(
              self.getReferences(),
              key=lambda ref: ref.getMd5Checksum())
          checksums = ''.join([ref.getMd5Checksum() for ref in references])
          md5checksum = hashlib.md5(checksums).hexdigest()
          return md5checksum
docs: Returns the MD5 checksum for this reference set. This checksum is
          calculated by making a list of `Reference.md5checksum` for all
          `Reference`s in this set. We then sort this list, and take the
          MD5 hash of all the strings concatenated together.