code: public SortedSet<Variable> support(final BDD bdd) {
      final int supportBDD = this.kernel.support(bdd.index());
      final Assignment assignment = createAssignment(supportBDD);
      assert assignment == null || assignment.negativeLiterals().isEmpty();
      return assignment == null ? new TreeSet<Variable>() : new TreeSet<>(assignment.positiveLiterals());
    }
docs: Returns all the variables that a given BDD depends on.
  @param bdd the BDD
  @return all the variables that the BDD depends on