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
  Vars(p <=> q,V) = Vars(p,V) \/ Vars(q,V)
proof
A1: the_right_side_of p <=> q = q by QC_LANG2:31;
  p <=> q is biconditional & the_left_side_of p <=> q = p by QC_LANG2:31,def 12
;
  hence thesis by A1,Th49;
end;
