'Advance the cursor in data buffer \'length\' bytes.'

def advance(self, length):

    new_position = (self._position + length)
    if ((new_position < 0) or (new_position > len(self._data))):
        raise Exception(('Invalid    advance    amount    (%s)    for    cursor.        Position=%s' % (length, new_position)))
    self._position = new_position
