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 Th10:
  for p,q being QC-formula of A holds still_not-bound_in(p '&' q) = (
  still_not-bound_in p) \/ (still_not-bound_in q)
proof
  let p,q be QC-formula of A;
  set pq = p '&' q;
A1: pq 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,Th9;
end;
