PeriodModulo​(AngouriMath.​Entity,​PeterO.​Numbers.​EInteger,​AngouriMath.​Entity.​Variable,​AngouriMath.​Entity.​Set.​SpecialSet)

The period, in x, of an expression's residue modulo
modulus: the modulus for a polynomial with whole coefficients,
and with a power a^e of a whole base coprime to the modulus among the terms --
7^n - 4^n modulo 3, 2^n + 1 modulo 7 -- the least common
multiple with the order of each base, since a^(e + k) = a^e modulo m where a^k = 1. Over the non-negative whole numbers only, where the powers are
whole; null for a base sharing a factor with the modulus, whose
powers repeat only eventually. Sullivan and Mackey's Ex 5.3.7 and §5.2.4 Try 3.