theorem Th77:
  Sum<*r1,r2*> = r1 + r2
proof
  reconsider s1=r1 as Element of REAL by XREAL_0:def 1;
  thus Sum<*r1,r2*> = Sum<*s1*> + r2 by Th74
    .= r1 + r2 by FINSOP_1:11;
end;
