@Article{Müller_Niehren:2000,
      AUTHOR = {Müller, Martin and Niehren, Joachim},
      TITLE = {Ordering Constraints over Feature Trees Expressed in Second-Order Monadic Logic},
      YEAR = {2000},
      JOURNAL = {Information and Computation},
      VOLUME = {159},
      NUMBER = {1-2},
      PAGES = {22-58},
      URL = {ftp://ftp.ps.uni-sb.de/pub/papers/ProgrammingSysLab/SWSJournal99.ps.gz},
      ABSTRACT = {The system FT$_leq$ of ordering constraints over feature trees has been introduced as an extension of the system FT of equality constraints over feature trees. While the first-order theory of FT is well understood, only few complexity and decidability results are known for fragments of the first-order theory of FT$_leq$. We introduce a new handle for such decidability questions by showing how to express ordering constraints over feature trees in second-order monadic logic (S2S or WS2S). Our relationship implies a new decidability result for feature logics, namely that the entailment problem of FT$_leq$ with existential quantifiers $phi_1models exists x_1ldotsexists x_n phi_2$ is decidable. We also show that this problem is PSPACE-hard even though the quantifier-free case can be solved in cubic time. To our knowledge, this is the first time that a non-trivial decidability result of feature logic is reduced to Rabins famous tree theorem.},
      ANNOTE = {COLIURL : Muller:2000:OCFa.pdf Muller:2000:OCFa.ps}
}