Class used as a key to store invariants in a MAP where their
equality depends on the invariant representing the same invariant
(i.e., their class is the same) and the same internal state (when
multiple invariants with the same class are possible)
Note that this is based on the Invariant type (i.e., class) and the
internal state and not on what ppt the invariant is in or what
variables it is over. Thus, invariants from different ppts are
the same if they represent the same type of invariant.