code: func (d *Dataset) Sort(name string, sortOption *SortOption) *Dataset {
  	ret := d.LocalSort(name, sortOption)
  	ret = ret.TreeMergeSortedTo(name, 1, 10)
  	return ret
  }
docs: // Sort sort on specific fields, default to the first field.
  // Required Memory: about same size as each partition.
  // example usage: Sort(Field(1,2)) means
  // sorting on field 1 and 2.