def lookup_path(self, mold_id_path, default=_marker):
        """
        For the given mold_id_path, look up the mold_id and translate
        that path to its filesystem equivalent.
        """

        fragments = mold_id_path.split('/')
        mold_id = '/'.join(fragments[:2])
        try:
            subpath = []
            for piece in fragments[2:]:
                if (sep in piece or (altsep and altsep in piece) or
                        piece == pardir):
                    raise KeyError
                elif piece and piece != '.':
                    subpath.append(piece)
            path = self.mold_id_to_path(mold_id)
        except KeyError:
            if default is _marker:
                raise
            return default

        return join(path, *subpath)
For the given mold_id_path, look up the mold_id and translate
        that path to its filesystem equivalent.