code: private Stack<Integer> createInitialCandidates(final List<Integer> variables) {
          for (final Integer var : variables) {
              if (isUPZeroLit(var)) {
                  final int backboneLit = mkLit(var, !this.model.get(var));
                  addBackboneLiteral(backboneLit);
              } else {
                  final boolean modelPhase = this.model.get(var);
                  if (isBothOrNegativeType() && !modelPhase || isBothOrPositiveType() && modelPhase) {
                      final int lit = mkLit(var, !modelPhase);
                      if (!this.config.isInitialUBCheckForRotatableLiterals() || !isRotatable(lit)) {
                          this.candidates.add(lit);
                      }
                  }
              }
          }
          return this.candidates;
      }
docs: Creates the initial candidate literals for the backbone computation.
  @param variables variables to test
  @return initial candidates