The input sets are publicly declared as either Set<T> or ReadonlySet<T>. And in TypeScript, Set extends ReadonlySet. Because it is functionally the same type but with extra properties for mutability.

When this library was initially written, each function exposed a generic S that extended ReadonlySet. This guaranteed that the return type would be the same as the input set types. And, when declared types were added to this library in v1.1.0, that typing paradigm S remained. But for internal use only, as only explicit cases for Set -> Set and ReadonlySet -> ReadonlySet are exposed in the distribution.

Because there are declared types for each function, that generic is no longer internally necessary. Set extends ReadonlySet. So each function can be internally typed with ReadonlySet. And the resultSets no longer need to be cast to S on return.