theorem
  (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 Th18
    .= (x1 - y1) + (x2 - y2) + (x3 - y3) by Th18;
end;
