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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def faces(self, sites, permutation=None):
 """ Returns the list of faces of this coordination geometry. Each face is given as a list of its vertices coordinates. """

        if permutation is None:
            coords = [site.coords for site in sites]
        else:
            coords = [sites[ii].coords for ii in permutation]
        return [[coords[ii] for ii in f] for f in self._faces]