theorem
  not x in still_not-bound_in p implies ( p '&' Ex(x,q) is valid iff Ex(
  x,p '&' q ) is valid )
proof
  assume not x in still_not-bound_in p;
  then (p '&' Ex(x,q)) <=> Ex(x,p '&' q) is valid by Th72;
  hence thesis by Lm15;
end;
