ProB can make use of symmetries induced by the use of deferred sets in B (and given sets in Z).

Warning: turning on symmetry reduction will also influence the way that
animation works. In other words, when executing an operation, the animator
may transfer you to an equivalent symmetric state (rather than to the one
you may have expected).

In the "Symmetry" menu of the "Preferences" menu you can choose the following:

"off": symmetry reduction is turned off.

"flood": This performs permutation flooding, whereby all permutations of a newly
         added state are automatically added (and marked as already treated).
         This does not reduce the number of states of the state space, but it
         may still considerably reduce the number of transitions and the number
         of states which need to be checked (for invariant violations and for
         computing outgoing transitions).

"nauty": This approach translates a B state into a graph and uses the nauty
         package in order to compute a canonical form of this graph.
         If two states have the same canonical form, they are considered
         equivalent and only one of them is explored. Warning: nauty sometimes
         can take a long time to compute the canonical form for complicated
         states (and in this case it will not respond to ProB's standard
         time-out mechanism).

"hash": This approach computes a symbolic hash value for every state, having
        the important property that two symmetric states have the same hash
        value. Note that this is an "approximate" method, as two non-symmetric
        states may also be mapped to the same hash value.
        This is typically the most efficient method.