code: public SortedSet<Literal> getCompleteBackbone() {
          final SortedSet<Literal> completeBackbone = new TreeSet<>();
          if (hasPositiveBackboneResult()) {
              completeBackbone.addAll(this.positiveBackbone);
          }
          if (hasNegativeBackboneResult()) {
              for (final Variable var : this.negativeBackbone) {
                  completeBackbone.add(var.negate());
              }
          }
          return Collections.unmodifiableSortedSet(completeBackbone);
      }
docs: Returns all literals of the backbone.  Positive backbone variables have positive polarity, negative
  backbone variables have negative polarity.
  @return the set of both positive and negative backbone variables as literals