code: public Formula dnf(final BDD bdd) {
      final List<Formula> ops = new LinkedList<>();
      for (final Assignment ass : this.enumerateAllModels(bdd))
        ops.add(ass.formula(this.f));
      return ops.isEmpty() ? this.f.falsum() : this.f.or(ops);
    }
docs: Returns a DNF formula for a given BDD.
  @param bdd the BDD
  @return the DNF for the formula represented by the BDD