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 Fixed(P!l) = { l.i : 1 <= i & i <= len l & l.i in fixed_QC-variables(A)}
by Th37;
