code: function providesCoverage( coverage, level, x, y ) {
      var rows,
          cols,
          i, j;
  
      if ( !coverage[ level ] ) {
          return false;
      }
  
      if ( x === undefined || y === undefined ) {
          rows = coverage[ level ];
          for ( i in rows ) {
              if ( rows.hasOwnProperty( i ) ) {
                  cols = rows[ i ];
                  for ( j in cols ) {
                      if ( cols.hasOwnProperty( j ) && !cols[ j ] ) {
                          return false;
                      }
                  }
              }
          }
  
          return true;
      }
  
      return (
          coverage[ level ][ x] === undefined ||
          coverage[ level ][ x ][ y ] === undefined ||
          coverage[ level ][ x ][ y ] === true
      );
  }
docs: @private
  @inner
  Returns true if the given tile provides coverage to lower-level tiles of
  lower resolution representing the same content. If neither x nor y is
  given, returns true if the entire visible level provides coverage.
  
  Note that out-of-bounds tiles provide coverage in this sense, since
  there's no content that they would need to cover. Tiles at non-existent
  levels that are within the image bounds, however, do not.
  
  @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}