//  [python]
def flip_interior_edges(self, is_flip_interior_edge):
        """"""
        if self._edges_cells is None:
            self._compute_edges_cells()

        interior_edges_cells = self._edges_cells[2]
        adj_cells = interior_edges_cells[is_flip_interior_edge].T

        edge_gids = self._edge_to_edge_gid[2][is_flip_interior_edge]
        adj_cells = interior_edges_cells[is_flip_interior_edge].T

        # Get the local ids of the edge in the two adjacent cells.
        # Get all edges of the adjacent cells
        ec = self.cells["edges"][adj_cells]
        # Find where the edge sits.
        hits = ec == edge_gids[None, :, None]
        # Make sure that there is exactly one match per cell
        assert n.all(n.sum(hits, axis=2) == 1)
        # translate to lids
        idx = n.empty(hits.shape, dtype=int)
        idx[..., 0] = 0
        idx[..., 1] = 1
        idx[..., 2] = 2
        lids = idx[hits].reshape((2, -1))

        #        3                   3
        #        A                   A
        #       /|\                 / \
        #      / | \               /   \
        #     /  |  \             /  1  \
        #   0/ 0 |   \1   ==>   0/_______\1
        #    \   | 1 /           \       /
        #     \  |  /             \  0  /
        #      \ | /               \   /
        #       \|/                 \ /
        #        V                   V
        #        2                   2
        #
        verts = n.array(
            [
                self.cells["nodes"][adj_cells[0], lids[0]],
                self.cells["nodes"][adj_cells[1], lids[1]],
                self.cells["nodes"][adj_cells[0], (lids[0] + 1) % 3],
                self.cells["nodes"][adj_cells[0], (lids[0] + 2) % 3],
            ]
        )

        self.edges["nodes"][edge_gids] = n.sort(verts[[0, 1]].T, axis=1)
        # No need to touch self.is_boundary_edge,
        # self.is_boundary_edge_individual; we're only flipping interior edges.

        # Do the neighboring cells have equal orientation (both node sets
        # clockwise/counterclockwise?
        equal_orientation = (
            self.cells["nodes"][adj_cells[0], (lids[0] + 1) % 3]
            == self.cells["nodes"][adj_cells[1], (lids[1] + 2) % 3]
        )

        # Set new cells
        self.cells["nodes"][adj_cells[0]] = verts[[0, 1, 2]].T
        self.cells["nodes"][adj_cells[1]] = verts[[0, 1, 3]].T

        # Set up new cells->edges relationships.
        previous_edges = self.cells["edges"][adj_cells].copy()

        i0 = n.ones(equal_orientation.shape[0], dtype=int)
        i0[~equal_orientation] = 2
        i1 = n.ones(equal_orientation.shape[0], dtype=int)
        i1[equal_orientation] = 2

        self.cells["edges"][adj_cells[0]] = n.column_stack(
            [
                n.choose((lids[1] + i0) % 3, previous_edges[1].T),
                n.choose((lids[0] + 2) % 3, previous_edges[0].T),
                edge_gids,
            ]
        )
        self.cells["edges"][adj_cells[1]] = n.column_stack(
            [
                n.choose((lids[1] + i1) % 3, previous_edges[1].T),
                n.choose((lids[0] + 1) % 3, previous_edges[0].T),
                edge_gids,
            ]
        )

        # update is_boundary_edge
        for k in range(3):
            self.is_boundary_edge[
                k, adj_cells
            ] = self.is_boundary_edge_individual[
                self.cells["edges"][adj_cells, k]
            ]

        # Update the edge->cells relationship. It doesn't change for the edge that was
        # flipped, but for two of the other edges.
        confs = [
            (0, 1, n.choose((lids[0] + 1) % 3, previous_edges[0].T)),
            (1, 0, n.choose((lids[1] + i0) % 3, previous_edges[1].T)),
        ]
        for conf in confs:
            c, d, edge_gids = conf
            num_adj_cells, edge_id = self._edge_gid_to_edge_list[edge_gids].T

            k1 = num_adj_cells == 1
            k2 = num_adj_cells == 2
            assert n.all(n.logical_xor(k1, k2))

            # outer boundary edges
            edge_id1 = edge_id[k1]
            assert n.all(
                self._edges_cells[1][edge_id1][:, 0] == adj_cells[c, k1]
            )
            self._edges_cells[1][edge_id1, 0] = adj_cells[d, k1]

            # interior edges
            edge_id2 = edge_id[k2]
            is_column0 = (
                self._edges_cells[2][edge_id2][:, 0] == adj_cells[c, k2]
            )
            is_column1 = (
                self._edges_cells[2][edge_id2][:, 1] == adj_cells[c, k2]
            )
            assert n.all(n.logical_xor(is_column0, is_column1))
            #
            self._edges_cells[2][edge_id2[is_column0], 0] = adj_cells[d, k2][
                is_column0
            ]
            self._edges_cells[2][edge_id2[is_column1], 1] = adj_cells[d, k2][
                is_column1
            ]

        # Schedule the cell ids for updates.
        update_cell_ids = n.unique(adj_cells.T.flat)
        # Same for edge ids
        k, edge_gids = self._edge_gid_to_edge_list[
            self.cells["edges"][update_cell_ids].flat
        ].T
        update_interior_edge_ids = n.unique(edge_gids[k == 2])

        self._update_cell_values(update_cell_ids, update_interior_edge_ids)
        return