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 Th39:
  Vars('not' p,V) = Vars(p,V)
proof
  set 9p = 'not' p;
A1: 9p is negative;
  then the_argument_of 9p = p by QC_LANG1:def 24;
  hence thesis by A1,Lm2;
end;
