theorem Th89:
  Sum(R1 + R2) = Sum R1 + Sum R2
proof
  i is Nat by TARSKI:1;
  hence Sum(R1 + R2) = addreal.(addreal$$R1,addreal$$R2) by SETWOP_2:35
    .= Sum R1 + Sum R2 by BINOP_2:def 9;
end;
