// To prevent the destruction of, say, branch entries.
    bool preserve_feat = true;

        // altars may be safely overwritten, ha!
        preserve_feat = false;

    // What's this supposed to achieve? (jpeg)
    // I mean, won't exits from the Abyss only turn up in the Abyss itself?

    dungeon_terrain_changed(c, feat, true, true, true);

    dungeon_terrain_changed(c, feat, preserve_feat, true, true);

    {

    }

    {

    }