Masking Of Heads; [n: Circuits For] Selecting Or Switching Of Heads Between Operative And Inoperative Functions [n: Or Between Different Operative Functions Or For Selection Between Operative Heads]; Masking Of Beams, E.g. Of Light Beams [n: Track Selection By Moving The Magnetic Head G9b/5.181] {g11b 15/12} Patents (Class G9B/15.016)