theorem Th8:
  -(x1 + x2 + x3) = -x1 + -x2 + -x3
proof
  thus -(x1+x2+x3) = 0*n -(x1+x2+x3) by RVSUM_1:33
    .= 0*n - (x1+x2) - x3 by RVSUM_1:39
    .= 0*n - x1 - x2 - x3 by RVSUM_1:39
    .= -x1 + -x2 + -x3 by RVSUM_1:33;
end;
