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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def generate_jwt(claims, priv_key=None, algorithm='PS512', lifetime=None, expires=None, not_before=None, jti_size=16, other_headers=None):
 """ Generate a JSON Web Token. :param claims: The claims you want included in the signature. :type claims: dict :param priv_key: The private key to be used to sign the token. Note: if you pass ``None`` then the token will be returned with an empty cryptographic signature and :obj:`algorithm` will be forced to the value ``none``. :type priv_key: `jwcrypto.jwk.JWK <https://jwcrypto.readthedocs.io/en/latest/jwk.html>`_ :param algorithm: The algorithm to use for generating the signature. ``RS256``, ``RS384``, ``RS512``, ``PS256``, ``PS384``, ``PS512``, ``ES256``, ``ES384``, ``ES512``, ``HS256``, ``HS384``, ``HS512`` and ``none`` are supported. :type algorithm: str :param lifetime: How long the token is valid for. :type lifetime: datetime.timedelta :param expires: When the token expires (if :obj:`lifetime` isn't specified) :type expires: datetime.datetime :param not_before: When the token is valid from. Defaults to current time (if ``None`` is passed). :type not_before: datetime.datetime :param jti_size: Size in bytes of the unique token ID to put into the token (can be used to detect replay attacks). Defaults to 16 (128 bits). Specify 0 or ``None`` to omit the JTI from the token. :type jti_size: int :param other_headers: Any headers other than "typ" and "alg" may be specified, they will be included in the header. :type other_headers: dict :rtype: unicode :returns: The JSON Web Token. Note this includes a header, the claims and a cryptographic signature. The following extra claims are added, per the `JWT spec <http://self-issued.info/docs/draft-ietf-oauth-json-web-token.html>`_: - **exp** (*IntDate*) -- The UTC expiry date and time of the token, in number of seconds from 1970-01-01T0:0:0Z UTC. - **iat** (*IntDate*) -- The UTC date and time at which the token was generated. - **nbf** (*IntDate*) -- The UTC valid-from date and time of the token. - **jti** (*str*) -- A unique identifier for the token. :raises: ValueError: If other_headers contains either the "typ" or "alg" header """

    header = {
        'typ': 'JWT',
        'alg': algorithm if priv_key else 'none'
    }

    if other_headers is not None:
        redefined_keys = set(header.keys()) & set(other_headers.keys())
        if redefined_keys:
            raise ValueError('other_headers re-specified the headers: {}'.format(', '.join(redefined_keys)))
        header.update(other_headers)

    claims = dict(claims)

    now = datetime.utcnow()

    if jti_size:
        claims['jti'] = base64url_encode(urandom(jti_size))

    claims['nbf'] = timegm((not_before or now).utctimetuple())
    claims['iat'] = timegm(now.utctimetuple())

    if lifetime:
        claims['exp'] = timegm((now + lifetime).utctimetuple())
    elif expires:
        claims['exp'] = timegm(expires.utctimetuple())

    if header['alg'] == 'none':
        signature = ''
    else:
        token = JWS(json_encode(claims))
        token.add_signature(priv_key, protected=header)
        signature = json_decode(token.serialize())['signature']

    return u'%s.%s.%s' % (
        base64url_encode(json_encode(header)),
        base64url_encode(json_encode(claims)),
        signature
    )