How to use the xgcm.duck_array_ops._apply_boundary_condition function in xgcm

# for every axis. If there is no connection data, fnum will not
            # be a key for self._connections.

            if fnum in self._connections:
                # it should always be a len 2 tuple
                face_connection = self._connections[fnum][0 if is_left_edge else 1]
            else:
                face_connection = None

            if (face_connection is None) or ignore_connections:
                # no connection: use specified boundary condition instead
                if self._facedim:
                    da_face = da.isel(**{self._facedim: slice(fnum, fnum + 1)})
                else:
                    da_face = da
                return _apply_boundary_condition(
                    da_face,
                    this_dim,
                    is_left_edge,
                    boundary=boundary,
                    fill_value=fill_value,
                )

            neighbor_fnum, neighbor_axis, reverse = face_connection

            # check for consistency
            if face_axis is None:
                assert neighbor_fnum is None

            # Build up a slice that selects the correct edge region for a
            # given face. We work directly with variables rather than
            # DataArrays in the hopes of greater efficiency, avoiding