theorem Th16:
  (x1+x2)+(y1+y2)=(x1+y1)+(x2+y2)
proof
  thus (x1+x2)+(y1+y2) = (x1+x2)+y1+y2 by RVSUM_1:15
    .= (x1+y1)+x2+y2 by RVSUM_1:15
    .= (x1+y1)+(x2+y2) by RVSUM_1:15;
end;
