code: public GroupReduceOperator<T, T> first(int n) {
  		if (n < 1) {
  			throw new InvalidProgramException("Parameter n of first(n) must be at least 1.");
  		}
  
  		return reduceGroup(new FirstReducer<T>(n));
  	}
docs: Returns a new set containing the first n elements in this grouped and sorted {@link DataSet}.
  @param n The desired number of elements for each group.
  @return A GroupReduceOperator that represents the DataSet containing the elements.