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 Th42:
  Vars(p '&' q,V) = Vars(p,V) \/ Vars(q,V)
proof
  set pq = p '&' q;
A1: p '&' q is conjunctive;
  then the_left_argument_of pq = p & the_right_argument_of pq = q by
QC_LANG1:def 25,def 26;
  hence thesis by A1,Lm2;
end;
