code: func (f *Field) RowTopN(n uint64, row *PQLRowQuery) *PQLRowQuery {
  	q := NewPQLRowQuery(fmt.Sprintf("TopN(%s,%s,n=%d)",
  		f.name, row.serialize(), n), f.index, nil)
  	return q
  }
docs: // RowTopN creates a TopN query with the given item count and row.
  // This variant supports customizing the row query.