“…Since the seminal work of Katriel [17], the combinatorial aspects of normal ordering arbitrary words in the creation and annihilation operators a † and a of a single-mode boson having the usual commutation relations [a, a † ] = aa † − a † a = 1, [a, a] = 0 and [a † , a † ] = 0 have been studied intensively since the seventies, see [14,15,17,18,[26][27][28][29][32][33][34], and references therein. From a more mathematical point of view, the consequences of the noncommutative calculus of operators has been considered, in particular, by Maslov [29].…”