code: @Pure
  	public boolean isReferenceCell(GridCell<P> cell) {
  		return !this.cells.isEmpty() && this.cells.get(0) == cell;
  	}
docs: Replies if the specified cell is the reference cell for the element.
  The reference cell is the cell where the element is supported to
  be attached when it is supposed by be inside only one cell.
  
  @param cell is the cell to test
  @return <code>true</code> if the specified cell is the reference cell;
  <code>false</code> otherwise.