'Abort a file transfer.  Uses out-of-band data.
This does not follow the procedure from the RFC to send Telnet
IP and Synch; that doesn\'t seem to work with the servers I\'ve
tried.  Instead, just send the ABOR command as OOB data.'

def abort(self):

    line = ('ABOR' + B_CRLF)
    if (self.debugging > 1):
        print ('*put    urgent*', self.sanitize(line))
    self.sock.sendall(line, MSG_OOB)
    resp = self.getmultiline()
    if (resp[:3] not in {'426', '225', '226'}):
        raise error_proto(resp)
    return resp
