'Use TOC to return subpath for given key, or raise a KeyError.'

def _lookup(self, key):

    try:
        if os.path.exists(os.path.join(self._path, self._toc[key])):
            return self._toc[key]
    except KeyError:
        pass
    self._refresh()
    try:
        return self._toc[key]
    except KeyError:
        raise KeyError(('No    message    with    key:    %s' % key))
