code: func (s *SortFilter) Num(n int) *SortFilter {
  	s.add(func(a, b string) int {
  		a1, a2 := column(a, n)
  		b1, b2 := column(b, n)
  		switch {
  		case a1 < b1:
  			return -1
  		case a1 > b1:
  			return +1
  		}
  
  		// Convert columns from strings to numbers.
  		a3, a4 := strconv.ParseFloat(a2, 64)
  		b3, b4 := strconv.ParseFloat(b2, 64)
  
  		if a4 != b4 {
  			// Errors sort after numbers.
  			if a4 != nil { // a had a parse error, b did not
  				return +1
  			}
  			// b had a parse error, a did not
  			return -1
  		}
  
  		switch {
  		case a3 < b3:
  			return -1
  		case a3 > b3:
  			return +1
  		}
  		return 0
  	})
  	return s
  }
docs: // Num sets the next sort key to sort by column n in numeric
  // order. Column 0 means the entire string. Items that do not have
  // column n sort to the front.  Items whose column n is not a number
  // sort to the end.