theorem Th64:
  ex x st not x in still_not-bound_in f
proof
  still_not-bound_in f is finite by Th62;
  then still_not-bound_in f <> bound_QC-variables(Al) by Th63;
  then
  not ( for b being object holds b in still_not-bound_in f iff
   b in bound_QC-variables(Al)) by TARSKI:2;
  then consider b such that
A1: not b in still_not-bound_in f and
A2: b in bound_QC-variables(Al);
  reconsider x = b as bound_QC-variable of Al by A2;
  take x;
  thus thesis by A1;
end;
