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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 validate_service_ticket(service, ticket, pgturl=None, renew=False, require_https=False):
 """ Validate a service ticket string. Return a triplet containing a ``ServiceTicket`` and an optional ``ProxyGrantingTicket``, or a ``ValidationError`` if ticket validation failed. """

    logger.debug("Service validation request received for %s" % ticket)

    # Check for proxy tickets passed to /serviceValidate
    if ticket and ticket.startswith(ProxyTicket.TICKET_PREFIX):
        raise InvalidTicketSpec('Proxy tickets cannot be validated with /serviceValidate')

    st = ServiceTicket.objects.validate_ticket(ticket, service, renew=renew, require_https=require_https)
    attributes = get_attributes(st.user, st.service)

    if pgturl is not None:
        logger.debug("Proxy-granting ticket request received for %s" % pgturl)
        pgt = ProxyGrantingTicket.objects.create_ticket(service, pgturl, user=st.user, granted_by_st=st)
    else:
        pgt = None
    return st, attributes, pgt