theorem
  not x in still_not-bound_in Ex(x,y,s) & not y in still_not-bound_in Ex
  (x,y,s)
proof
  not y in still_not-bound_in Ex(y,s) by Th6;
  then
A1: not y in still_not-bound_in Ex(x,Ex(y,s)) by Th6;
  not x in still_not-bound_in Ex(x,Ex(y,s)) by Th6;
  hence thesis by A1,QC_LANG2:14;
end;
