'Return whether this nonce is present, and if it is, then
remove it from the set.
str -> bool'

def txn_useNonce(self, server_url, timestamp, salt):

    if (abs((timestamp - time.time())) > nonce.SKEW):
        return False
    try:
        self.db_add_nonce(server_url, timestamp, salt)
    except self.exceptions.IntegrityError:
        return False
    else:
        return True
