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 Th37:
  for P being QC-pred_symbol of k, A for l being QC-variable_list of
  k, A holds Vars(P!l,V) = variables_in(l, V) &
  Vars((P!l),V) = { l.i : 1 <= i & i
  <= len l & l.i in V }
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,Th36;
end;
