Set() object implements a finite set of Reals, not of arbitrary objects. See the wiki documentation for the Set Object for details.Sets are intended for use in the Interval context to handle things like the intersection of the intervals
(1,3] and [3,5) (without them, you would have to use something like [3,3]).It is not an error for the Set object to complain about a set containing a point (or other object classes), since they are sets of reals.
If you want to implement a more general Set class that is a set of arbitrary objects, that would be great. We would welcome the contribution.