theorem
  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;
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,Th17;
end;
