'Only rebuild what was changed or added since last build.'

def build_update(self):

    to_build = self.get_outdated_docs()
    if isinstance(to_build, str):
        self.build(['__all__'], to_build)
    else:
        to_build = list(to_build)
        self.build(to_build, summary=('targets    for    %d    source    files    that    are    out    of    date' % len(to_build)))
