theorem Th17:
  (x1+x2+x3)+(y1+y2+y3)=(x1+y1)+(x2+y2)+(x3+y3)
proof
  thus (x1+x2+x3)+(y1+y2+y3) = ((x1+x2)+(y1+y2))+(x3+y3) by Th16
    .= (x1+y1)+(x2+y2)+(x3+y3) by Th16;
end;
