code: function isCovered( coverage, level, x, y ) {
      if ( x === undefined || y === undefined ) {
          return providesCoverage( coverage, level + 1 );
      } else {
          return (
               providesCoverage( coverage, level + 1, 2 * x, 2 * y ) &&
               providesCoverage( coverage, level + 1, 2 * x, 2 * y + 1 ) &&
               providesCoverage( coverage, level + 1, 2 * x + 1, 2 * y ) &&
               providesCoverage( coverage, level + 1, 2 * x + 1, 2 * y + 1 )
          );
      }
  }
docs: @private
  @inner
  Returns true if the given tile is completely covered by higher-level
  tiles of higher resolution representing the same content. If neither x
  nor y is given, returns true if the entire visible level is covered.
  
  @param {Object} coverage - A '3d' dictionary [level][x][y] --> Boolean.
  @param {Number} level - The resolution level of the tile.
  @param {Number} x - The X position of the tile.
  @param {Number} y - The Y position of the tile.
  @returns {Boolean}