'Build a single channel, usually called by build()
@param key: the channel key
@param attr: HTML attributes for the channel
@return: the XML for the channel, usually a DIV instance'

def build_channel(self, key, **attr):

    widgets = default = XML('&nbsp;')
    channel = self.channels.get(key)
    if (channel is not None):
        widgets = [w for w in channel]
        if (not widgets):
            widgets = default
    return DIV(widgets, **attr)
