Subring is a type that stores information associated to a subring of a polynomial ring, such as a set of subring generators and a reference to the polynomial ring it is contained in. An instance of a Subring is constructed with the function subring.
Every instance of Subring is guaranteed to have the following keys:

ambientRing: The polynomial ring that contains the subring instance's generators.

generators: A onerow matrix, the generators of the subring.

isSAGBI: A boolean that is false by default. This flag is only set to true in subring instances resulting from a Sagbi computation that terminated successfully. If this is true, the generators are a Sagbi basis.

presentation: An instance of the PresRing type associated with the subring's generators.

cache: Contains unspecified information. The contents of the cache may effect performance, but should never effect the result of a computation.