TypeStateDomain with multiple underlying typestate properties

[khatchad]

From https://github.com/tech-srl/safe/blob/master/com.ibm.safe.typestate/src/com/ibm/safe/typestate/core/TypeStateDomain.java:

/**
 * The domain of factoids which the IFDS solver operates on.
 * 
 * Currently, a typestate domain assumes a single underlying typestate property.
 * 
 * @author Eran Yahav (yahave)
 * @author Stephen Fink
 */
public abstract class TypeStateDomain //...

It would be useful to have a domain assume multiple underlying typestate properties, especially for performance reasons.