  We show that the query containment problem for monadic datalog on finite
unranked labeled trees can be solved in 2-fold exponential time when (a)
considering unordered trees using the axes child and descendant, and when (b)
considering ordered trees using the axes firstchild, nextsibling, child, and
descendant. When omitting the descendant-axis, we obtain that in both cases the
problem is EXPTIME-complete.
