'Returns the next completion for txt, or None if there is no completion'

def complete(self, txt):

    if (not self.lookup):
        self.lookup = []
        for (wid, window) in self.qtile.windowMap.items():
            if (window.group and window.name.lower().startswith(txt)):
                self.lookup.append((window.name, wid))
        self.lookup.sort()
        self.offset = (-1)
        self.lookup.append((txt, txt))
    self.offset += 1
    if (self.offset >= len(self.lookup)):
        self.offset = 0
    ret = self.lookup[self.offset]
    self.thisfinal = ret[1]
    return ret[0]
