// A monster can only move through or inhabit permanent rock if that's
        // exactly what it's asking for.
        if (actual_grid == DNGN_PERMAROCK_WALL
            || actual_grid == DNGN_CLEAR_PERMAROCK_WALL)
        {
            return (grid_wanted == DNGN_PERMAROCK_WALL
                    || grid_wanted == DNGN_CLEAR_PERMAROCK_WALL);
        }