<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def facets_boundary(self):
 """ Return the edges which represent the boundary of each facet Returns --------- edges_boundary : sequence of (n, 2) int Indices of self.vertices """

        # make each row correspond to a single face
        edges = self.edges_sorted.reshape((-1, 6))
        # get the edges for each facet
        edges_facet = [edges[i].reshape((-1, 2)) for i in self.facets]
        edges_boundary = np.array([i[grouping.group_rows(i, require_count=1)]
                                   for i in edges_facet])
        return edges_boundary