code: private boolean isUnit(final int lit, final MSClause clause) {
          for (int i = 0; i < clause.size(); ++i) {
              final int clauseLit = clause.get(i);
              if (lit != clauseLit && this.model.get(var(clauseLit)) != sign(clauseLit)) {
                  return false;
              }
          }
          return true;
      }
docs: Tests the given literal whether it is unit in the given clause.
  @param lit    literal to test
  @param clause clause containing the literal
  @return {@code true} if the literal is unit, {@code false} otherwise