public static CoveredTiles merge(CoveredTiles a, CoveredTiles b) {
      if (a.z != b.z) {
        throw new IllegalArgumentException("Cannot combine CoveredTiles with different zoom levels ");
      }
      return new CoveredTiles(RoaringBitmap.or(a.bitmap, b.bitmap), a.z);
    }
Returns the union of tiles covered by {@code a} and {@code b}.

@throws IllegalArgumentException if {@code a} and {@code b} have different zoom levels.
planetiler-core/src/main/java/com/onthegomap/planetiler/render/TiledGeometry.java