code: protected void unmark(final int level) {
          assert level <= this.seen.size();
          while (level < this.seen.size()) {
              final int lit = this.seen.back();
              this.seen.pop();
              final CLVar v = var(lit);
              assert v.mark() == sign(lit);
              v.setMark(0);
          }
      }
docs: Unmarks all variables up to a given level
  @param level the level