sage: Set(ZZ)
Set of elements of Integer Ring
sage: Set(RealSet(-oo,oo))
Set of elements of (-oo, +oo)
A bit more tricky are the reals:
sage: Set(RealSet(-oo,oo))
Set of elements of (-oo, +oo)
sage: R = Set(RealSet(-oo,oo))
sage: R.an_element()
(-oo, +oo)
sage: RealSet(-oo,+oo).an_element()
...
SignError: cannot add infinity to minus infinity
sage: R = Set(RealSet(-oo,oo))
sage: R.an_element()
(-oo, +oo)
In passing, we may also note that
sage: RealSet(-oo,+oo).an_element()
...
SignError: cannot add infinity to minus infinity