  In the article 'Recursive queries on trees and data trees' (ICDT'13),
Abiteboul et al., asked whether the containment problem for monadic datalog
over unordered unranked labeled trees using the child relation and the
descendant relation is decidable. This note gives a positive answer to this
question, as well as an overview of the relative expressive power of monadic
datalog on various representations of unranked trees.
