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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 face_subset(self, face_index):
 """ Given a mask of face indices, return a sliced version. Parameters face_index: (n,) int, mask for faces (n,) bool, mask for faces Returns visual: ColorVisuals object containing a subset of faces. """

        if self.defined:
            result = ColorVisuals(
                face_colors=self.face_colors[face_index])
        else:
            result = ColorVisuals()

        return result