@Override
    public String toString() {
      return "CoveredTiles{z=" + z + ", tiles=" + FORMAT.integer(bitmap.getCardinality()) + ", storage=" +
        FORMAT.storage(bitmap.getSizeInBytes()) + "B}";
    }
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