code: public SortedMap<Variable, Integer> variableProfile(final BDD bdd) {
      final int[] varProfile = this.kernel.varProfile(bdd.index());
      final SortedMap<Variable, Integer> profile = new TreeMap<>();
      for (int i = 0; i < varProfile.length; i++) {
        profile.put(this.idx2var.get(i), varProfile[i]);
      }
      return profile;
    }
docs: Returns how often each variable occurs in the given BDD.
  @param bdd the BDD
  @return how often each variable occurs in the BDD