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

theorem Th22:
  -- {} = {}
proof
  assume -- {} <> {};
  then consider x be object such that
  A1: x in --{} by XBOOLE_0:def 1;
  ex y st y in {} & x=-y by A1,Def4;
  hence thesis;
end;
