code: protected int unmarkFrames() {
          final int res = this.frames.size();
          while (!this.frames.empty()) {
              final CLFrame f = this.control.get(this.frames.back());
              this.frames.pop();
              assert f.mark();
              f.setMark(false);
          }
          return res;
      }
docs: Unmarks all frames.
  @return the number of unmarked frames