theorem
  not x in still_not-bound_in All(x,y,s) & not y in still_not-bound_in
  All(x,y,s)
proof
  not y in still_not-bound_in All(y,s) by Th5;
  then
A1: not y in still_not-bound_in All(x,All(y,s)) by Th5;
  not x in still_not-bound_in All(x,All(y,s)) by Th5;
  hence thesis by A1,QC_LANG2:14;
end;
