An interface used by url.viewhere. When this interface is remembered
above a url.viewhere embedded in a page, and the url to the current page
is rendered, this object will be consulted for additional manipulations
to perform on the url object before returning it.
Return an iterator which yields a series of (command, args, kw)
triples, where 'command' is a string, indicating which url method to
call, 'args' is a list indicating the arguments to be passed to this
method, and 'kw' is a dict of keyword arguments to pass to this
method.