theorem Th5:
  OSSign S0 is discrete op-discrete
proof
  set s = OSSign S0;
  set ol = the Overloading of s;
  the carrier of S0 = the carrier of OSSign S0 & id the carrier of S0 =
  the InternalRel of OSSign S0 by Def5;
  hence OSSign S0 is discrete by ORDERS_3:def 1;
  the Overloading of OSSign S0 = id the carrier' of S0 by Def5;
  then for x,y be OperSymbol of s st x ~= y holds x = y by RELAT_1:def 10;
  hence thesis by Th3;
end;
