 The author would like to thank: Gregory Landini, Dick Schmitt, Franz Fritsche, Rafal Urbaniak, Adam Trybus, Pawel Manczyk, Kenneth Blackwell, and Dirk Schlimm for corrections to this entry. Axel Boldt studied the most recent revisions and found numerous mathematical errors and among other insights pointed out the use of double dots for conjunction at [∗10·55] and the oddity involved in the PM notions of the domain and range of a function. 