# Read
var csvfile = %f'data.csv';
var fh = csvfile.open_r;
var header = fh.line.trim_end.split(',');
var csv = fh.lines.map { .trim_end.split(',').map{.to_num} };
fh.close;

# Write
var out = csvfile.open_w;
out.say([header..., 'SUM'].join(','));
csv.each { |row| out.say([row..., row.sum].join(',')) };
out.close;