theorem Th45:
  p is disjunctive implies Vars(p,V) = Vars(the_left_disjunct_of p
  ,V) \/ Vars(the_right_disjunct_of p,V)
proof
  set p1 = the_left_disjunct_of p;
  set p2 = the_right_disjunct_of p;
  assume p is disjunctive;
  then p = p1 'or' p2 by QC_LANG2:37;
  then p = 'not'('not' p1 '&' 'not' p2) by QC_LANG2:def 3;
  hence Vars(p,V) = Vars('not' p1 '&' 'not' p2, V) by Th39
    .= Vars('not' p1, V) \/ Vars('not' p2, V) by Th42
    .= Vars(p1, V) \/ Vars('not' p2, V) by Th39
    .= Vars(the_left_disjunct_of p,V) \/ Vars(the_right_disjunct_of p,V) by
Th39;
end;
