theorem
  Vars(Ex(x,p), V) = Vars(p,V)
proof
  Ex(x,p) = 'not' All(x,'not' p) by QC_LANG2:def 5;
  hence Vars(Ex(x,p), V) = Vars(All(x,'not' p), V) by Th39
    .= Vars('not' p, V) by Th44
    .= Vars(p, V) by Th39;
end;
