
theorem
  for F being Field holds for a,b,c being Element of F
  holds a+b = c iff osf(F).(c,a) = b
proof
  let F be Field;
  let a,b,c be Element of F;
  set d=0.F;
  c+d = c & osf(F).(b,d) = b by Th15,REALSET2:2;
  hence thesis by Th13;
end;
