Opus number developed other catalogue-number systems; among them the "Bach-Werke-Verzeichnis" (BWV-number), and the "Köchel-Verzeichnis" (K- and KV -numbers) with which are organised the works of Johann Sebastian Bach and Wolfgang Amadeus Mozart, respectively. In the classical period, the Latin word "opus" ("work", "labour") was used to identify, list, and catalogue a work of art. By the 15th and 16th centuries, the word "opus" was used by Italian composers to denote a specific musical composition, and by German composers for collections of music. In compositional practise, numbering musical works in chronological order dates from 17th century Italy, especially Venice. In common usage, the
Stirling numbers of the second kind for Stirling numbers of the second kind. The brace notation formula_2 was used by Imanuel Marx and Antonio Salmeri in 1962 for variants of these numbers. This led Knuth to use it, as shown here, in the first volume of "The Art of Computer Programming" (1968). However, according to the third edition of "The Art of Computer Programming", this notation was also used earlier by Jovan Karamata in 1935. The notation "S"("n", "k") was used by Richard Stanley in his book "Enumerative Combinatorics". Since the Stirling number formula_15 counts set partitions of an "n"-element set into "k" parts, the sum