theorem Th13:
  for p being QC-formula of A st p is disjunctive holds
  still_not-bound_in p = (still_not-bound_in the_left_disjunct_of p) \/ (
  still_not-bound_in the_right_disjunct_of p)
proof
  let p be QC-formula of A;
  set p1 = the_left_disjunct_of p;
  set p2 = the_right_disjunct_of p;
  assume p is disjunctive;
  then p = (the_left_disjunct_of p) 'or' (the_right_disjunct_of p) by
QC_LANG2:37;
  then p = 'not'('not' p1 '&' 'not' p2) by QC_LANG2:def 3;
  then still_not-bound_in p = still_not-bound_in 'not' p1 '&' 'not' p2 by Th7
    .= (still_not-bound_in 'not' p1) \/ (still_not-bound_in 'not' p2) by Th10
    .= (still_not-bound_in p1) \/ (still_not-bound_in 'not' p2) by Th7
    .= (still_not-bound_in p1) \/ (still_not-bound_in p2) by Th7;
  hence thesis;
end;
