Constant PERIOD_FORM 

const PERIOD_FORM: &str = "a count followed by a unit letter D, W, M, Y or T, such as 3M or -1Y, or descending segments of one family such as 1Y3M or 2W3D, spanning at most 200 years";

The shape a tenor code must have, named on rejection.

Open on the multiplier — any integer is a period — so there is no set to enumerate; what a caller gets wrong is the unit letter or the order.