'Validates that the nonce has not been used before.
Per `Section 3.3`_ of the spec.
"A nonce is a random string, uniquely generated by the client to allow
the server to verify that a request has never been made before and
helps prevent replay attacks when requests are made over a non-secure
channel.  The nonce value MUST be unique across all requests with the
same timestamp, client credentials, and token combinations."
.. _`Section 3.3`: http://tools.ietf.org/html/rfc5849#section-3.3'

def validate_timestamp_and_nonce(self, client_key, timestamp, nonce, request_token=None, access_token=None):

    raise NotImplementedError('Subclasses    must    implement    this    function.')
