theorem Th26:
  f is A-measurable & g is A-measurable implies f+g is A-measurable
proof
  assume f is A-measurable & g is A-measurable;
  then R_EAL f is A-measurable & R_EAL g is A-measurable;
  then R_EAL f + R_EAL g is A-measurable by MESFUNC2:7;
  then R_EAL(f+g) is A-measurable by Th23;
  hence thesis;
end;
