A method for use with an aggregation operation includes a sorting pass and a merging pass. Said locking system including a tongue and a groove for vertical locking of two joint short edges and for horizontal joining a locking groove formed in the underside of a first one of the short joint edges, and a portion projecting from the second short joint edge having a locking element cooperating with the locking groove. Each of the high refractive index portions 303 is generally interposed between a pair of holes 304 which are in symmetric positions with respect to the center axis.