code: @Pure
  	public int indexOf(P element) {
  		int idx;
  		int globalIdx = 0;
  		for (final GridCell<P> cell : getGridCells()) {
  			idx = cell.indexOf(element);
  			if (idx != -1) {
  				return globalIdx + idx;
  			}
  			globalIdx += cell.getReferenceElementCount();
  		}
  		return -1;
  	}
docs: Replies the index of the specified element.
  
  @param element the element.
  @return the index of the specified element or {@code -1} if it was not found.