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 Th40:
  Vars(FALSUM(A),V) = {}
proof
  FALSUM(A) = 'not' VERUM(A) by QC_LANG2:def 1;
  hence Vars(FALSUM(A),V) = Vars(VERUM(A),V) by Th39 .= {} by Lm2;
end;
