docs: public InstanceSet[] split(float percent) {
          shuffle();
          int length = this.size();
          InstanceSet[] sets = new InstanceSet[2];
          sets[0] = new InstanceSet(pipes, factory);
          sets[1] = new InstanceSet(pipes, factory);
          int idx = (int) Math.round(percent*length);
          sets[0].addAll(subList(0, idx));
          if(idx+1<length)            
              sets[1].addAll(subList(idx+1, length));
          return sets;
      }