code: public UnsortedGrouping<T> groupBy(int... fields) {
  		return new UnsortedGrouping<T>(this, new Keys.FieldPositionKeys<T>(fields, getType(), false));
  	}
docs: Groups a {@link Tuple} {@link DataSet} using field position keys.<br/>
  <b>Note: Field position keys only be specified for Tuple DataSets.</b></br>
  The field position keys specify the fields of Tuples on which the DataSet is grouped.
  This method returns an {@link UnsortedGrouping} on which one of the following grouping transformation
  can be applied.
  <ul>
  <li>{@link UnsortedGrouping#sortGroup(int, eu.stratosphere.api.common.operators.Order)} to get a {@link SortedGrouping}.
  <li>{@link Grouping#aggregate(Aggregations, int)} to apply an Aggregate transformation.
  <li>{@link Grouping#reduce(ReduceFunction)} to apply a Reduce transformation.
  <li>{@link Grouping#reduceGroup(GroupReduceFunction)} to apply a GroupReduce transformation.
  </ul>
  
  @param fields One or more field positions on which the DataSet will be grouped.
  @return A Grouping on which a transformation needs to be applied to obtain a transformed DataSet.
  
  @see Tuple
  @see Grouping
  @see UnsortedGrouping
  @see SortedGrouping
  @see AggregateOperator
  @see ReduceOperator
  @see GroupReduceOperator
  @see DataSet