code: def useNonce(self, server_url, timestamp, salt):
          
          if abs(timestamp - time.time()) > nonce.SKEW:
              return False
  
          if server_url:
              proto, rest = server_url.split('://', 1)
          else:
              # Create empty proto / rest values for empty server_url,
              # which is part of a consumer-generated nonce.
              proto, rest = '', ''
  
          domain = _filenameEscape(rest.split('/', 1)[0])
          url_hash = _safe64(server_url)
          salt_hash = _safe64(salt)
  
          filename = '%08x-%s-%s-%s-%s' % (timestamp, proto, domain,
                                           url_hash, salt_hash)
  
          filename = os.path.join(self.nonce_dir, filename)
          try:
              fd = os.open(filename, os.O_CREAT | os.O_EXCL | os.O_WRONLY, 0200)
          except OSError, why:
              if why.errno == EEXIST:
                  return False
              else:
                  raise
          else:
              os.close(fd)
              return True
docs: Return whether this nonce is valid.
  
          str -> bool