code: public void addEquality(SVar<T> s1, SVar<T> s2) {
  	this.addInclusion(s1, s2);
  	this.addInclusion(s2, s1);
      }
docs: Adds two mutual inclusion constraints to reflect the fact that
  the sets <code>s1</code> and <code>s2</code> should be
  equal.