code: public LNGIntVector upZeroLiterals() {
          final LNGIntVector upZeroLiterals = new LNGIntVector();
          for (int i = 0; i < this.trail.size(); ++i) {
              final int lit = this.trail.get(i);
              if (var(lit).level() > 0) {
                  break;
              } else {
                  upZeroLiterals.push(lit);
              }
          }
          return upZeroLiterals;
      }
docs: Returns the unit propagated literals on level zero.
  @return unit propagated literal on level zero