code: def hashes(self):
          
          hashes = set()
          if (self.resources is not None):
              for resource in self:
                  if (resource.md5 is not None):
                      hashes.add('md5')
                  if (resource.sha1 is not None):
                      hashes.add('sha-1')
                  if (resource.sha256 is not None):
                      hashes.add('sha-256')
          return(hashes)
docs: Return set of hashes uses in this resource_list.