'Switch to the next window in current column'

def cmd_down(self):

    c = self.current_column()
    if (c is None):
        return
    ridx = c['rows'].index(self.current_window)
    if (ridx == (len(c['rows']) - 1)):
        if (c['mode'] != 'split'):
            ridx = 0
    else:
        ridx += 1
    client = c['rows'][ridx]
    self.group.focus(client)
