“…This can be traced back to the elegant approach proposed by Schwinger [15,16,17], who clearly recognized that the expansion of arbitrary operators in terms of certain operator basis was the crucial mathematical concept in setting such a grid. These ideas have been rediscovered and developed further by several authors [18,19,20,21,22,23,24], although the contributions of Wootters [25,26,27,28] and Galetti and coworkers [29,30,31] are worth stressing.…”