theorem Th28:
  {} in Toler_on_subsets(X)
proof
  set F = the set of all Toler(Y) where Y is Subset of X;
  {} c= X;
  then
A1: Toler({}) in F;
  {} in Toler({}) by Def15,TOLER_1:14;
  hence thesis by A1,TARSKI:def 4;
end;
