private Summary mergeIn(Summary other) {
      for (int z = PlanetilerConfig.MIN_MINZOOM; z <= PlanetilerConfig.MAX_MAXZOOM; z++) {
        byTile.get(z).mergeIn(other.byTile.get(z));
      }
      for (int z = PlanetilerConfig.MIN_MINZOOM; z <= PlanetilerConfig.MAX_MAXZOOM; z++) {
        var ourMap = byLayer.get(z);
        var theirMap = other.byLayer.get(z);
        theirMap.forEach((layer, stats) -> ourMap.merge(layer, stats, Cell::combine));
      }
      return this;
    }
Returns the minimum zoom a specific layer appears at in the tileset.
planetiler-core/src/main/java/com/onthegomap/planetiler/util/TilesetSummaryStatistics.java