 Complete the following phrase:  Some association football clubs have started doing this as squad numbers have become common. AS Roma, AC Milan, Ajax, Internazionale, Napoli, Manchester City, Lens, Lyon, Nantes and Swansea City have all retired shirt numbers; Milan retiring Franco Baresis 6 shirt and Paolo Maldinis 3 shirt with the caveat that one of Maldinis sons can wear the shirt if they play professionally for the club. Swansea retired the shirt number of Besian Idrizaj after he died in his sleep, of a suspected heart attack, at his family home in Linz, Austria. Manchester City, Lens and Lyon all retired the shirt number of Marc-Vivien Foé after his death on the field in the 