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;
