theorem Th30:
  a c= X implies id a in Toler_on_subsets(X)
proof
  set F = the set of all Toler(Y) where Y is Subset of X;
  assume a c= X;
  then
A1: Toler(a) in F;
  id a in Toler(a) by Def15;
  hence thesis by A1,TARSKI:def 4;
end;
