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 Th7:
  for p being QC-formula of A holds still_not-bound_in 'not' p =
  still_not-bound_in p
proof
  let p be QC-formula of A;
A1: 'not' p is negative;
  then the_argument_of 'not' p = p by QC_LANG1:def 24;
  hence thesis by A1,Th6;
end;
