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

theorem Th8:
  L_(-x) = --R_x & R_(-x) = --L_x
proof
  -x = [-- R_x, --L_x] by Th7;
  hence thesis;
end;
