Gilbert E. N.. N-terminal switching circuits. The Bell System technical journal, vol. 30 (1951), pp. 668–688.

where a is any w-position switch. The author's system is prepositional algebra rather than prepositional calculus, since only equivalences are asserted. It is, moreover, ordinary two-valued prepositional algebra, since every expression of the algebra has (in effect) one or other of two truth-values, truth or falsehood. And it is indeed the reviewer's impression that the author's method of dealing with multiple-position switches can be expected to work more smoothly than proposals which have sometimes been made to use many-valued propositional calculus for this. Errata: in line 14 from the bottom of page 674, it seems probable that ai was meant to have a bar over it; at several places on page 675, a; is evidently a misprint for a\ , and ce a misprint for ce. ALONZO CHURCH