reserve A,B,C for Ordinal,
        o for object,
        x,y,z,t,r,l for Surreal,
        X,Y for set;

theorem Th10:
  x <= y iff -y <= -x
proof
  set Bx=born x,By=born y,B=Bx\/By;
  A1:Day Bx c= Day B & Day By c= Day B by SURREAL0:35,XBOOLE_1:7;
  x in Day Bx & y in Day By by SURREAL0:def 18;
  hence thesis by A1,Lm1;
end;
