code: protected boolean markFrame(final int lit) {
          final int currentlevel = var(lit).level();
          final CLFrame frame = this.control.get(currentlevel);
          if (frame.mark()) { return false; }
          frame.setMark(true);
          this.frames.push(currentlevel);
          return true;
      }
docs: Marks the frame for a given literal's decision level.
  @param lit the literal
  @return {@code true} if the frame was newly marked, {@code false} if the frame was already marked