code: public Dataframe getSubset(FlatDataList idsCollection) {
          Dataframe d = new Dataframe(configuration);
  
          for(Object id : idsCollection) {
              d.add(get((Integer)id));
          }
          return d;
      }
docs: It generates and returns a new Dataframe which contains a subset of this Dataframe.
  All the Records of the returned Dataframe are copies of the original Records.
  The method is used for k-fold cross validation and sampling. Note that the
  Records in the new Dataframe have DIFFERENT ids from the original ones.
  
  @param idsCollection
  @return