code: private List<SpriteTiled> getRasters(Integer sheet)
      {
          return rasterSheets.computeIfAbsent(sheet, s ->
          {
              final List<SpriteTiled> rasters = new ArrayList<>(RasterImage.MAX_RASTERS);
              rasterSheets.put(s, rasters);
              return rasters;
          });
      }
docs: Get the associated raster sheets for a sheet number. Create it if needed.
  
  @param sheet The sheet number
  @return The raster sheets collection found.