'Returns the next client after \'win\' in layout,
or None if there is no such client'

def focus_next(self, win):

    for (idx, col) in enumerate(self.columns):
        if (win in col):
            nxt = col.focus_next(win)
            if nxt:
                return nxt
            else:
                break
    if ((idx + 1) < len(self.columns)):
        return self.columns[(idx + 1)].focus_first()
