If m, n are integers, “m = n” will mean “m = n (mod 2 ).“ The cardinal number of a set A will be denoted by |A|. The set whose elements are a1 a2, … , an will be denoted by {a1, a2, … , an}. The empty set will be denoted by Ø. If A, B, C are sets, A — B will denote the set of those elements of A which do not belong to B, and A — B — C will denote (A — B)— C. The expression Σε∈Af(ε) will be denoted by f. A. The statements “f = g on A,” “f ≡ g on A” will mean that f(ε) = g(ε) or f(ε) ≡ g(ε) respectively for every ε ∈ A.