reserve i,k for Nat;
reserve A for QC-alphabet;
reserve x for bound_QC-variable of A;
reserve a for free_QC-variable of A;
reserve p,q for Element of QC-WFF(A);
reserve l for FinSequence of QC-variables(A);
reserve P,Q for QC-pred_symbol of A;
reserve V for non empty Subset of QC-variables(A);
reserve s,t for QC-symbol of A;

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;
