code: public boolean contains(Integer sheet, int number)
      {
          for (final TileRef current : tiles)
          {
              if (current.getSheet().equals(sheet) && current.getNumber() == number)
              {
                  return true;
              }
          }
          return false;
      }
docs: Check if tile is contained by the group.
  
  @param sheet The sheet number.
  @param number The tile number.
  @return <code>true</code> if part of the group, <code>false</code> else.