theorem
  for P being QC-pred_symbol of k,A for l being QC-variable_list of k, A
  holds still_not-bound_in (P!l) = still_not-bound_in l
proof
  let P be QC-pred_symbol of k, A;
  let l be QC-variable_list of k, A;
A1: P!l is atomic;
  then the_arguments_of (P!l) = l by QC_LANG1:def 23;
  hence thesis by A1,Th4;
end;
