This week, I had been working on implementaion of Union and Contains in this PR.
Our Set::contains function has been changed from :

virtual bool contains(const RCP<const Basic> &a) const = 0;

to

virtual RCP<const Boolean> contains(const RCP<const Basic> &a) const = 0;

And these functions:

virtual bool is_subset(const RCP<const Set> &o) const = 0;
virtual bool is_proper_subset(const RCP<const Set> &o) const = 0;
virtual bool is_superset(const RCP<const Set> &o) const = 0;
virtual bool is_proper_superset(const RCP<const Set> &o) const = 0;

have been changed to:

bool is_subset(const RCP<const Set> &o) const
{
    return eq(*this->set_intersection(o), *this);
}
bool is_proper_subset(const RCP<const Set> &o) const
{
    return not eq(*this, *o) and this->is_subset(o);
}
bool is_superset(const RCP<const Set> &o) const
{
    return o->is_subset(rcp_from_this_cast<const Set>());
}
bool is_proper_superset(const RCP<const Set> &o) const
{
    return not eq(*this, *o) and this->is_superset(o);
}

depending solely on set_intersection.

Then the SymEngine::set_union(const set_set &in, bool solve = true) has been defined which will create a Union object with in if it is not solvable, or will do union operation on all of them.
Our Union class has std::set of Set to store the different Set containers which can’t be unified into one single container.
Apart from it, the set_intersection ans set_union virtual functions have been restructured to handle other Set types case by case.