The first-order theory of ordering constraints over feature
trees
Author: Martin Müller and Joachim Niehren and Ralf Treinen
Editor:
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. We investigate the first-order theory of FT$_\leq$
and its fragments in detail, both over
finite trees and over possibly infinite trees. We prove that the
first-order theory of FT$_\leq$ is
undecidable, in contrast to the first-order theory of FT which
is well-known to be decidable. We show
that the entailment problem of FT$_\leq$
with existential quantification is PSPACE-complete. So far, this
problem has been shown decidable, coNP-hard in case of finite trees,
PSPACE-hard in case of arbitrary trees, and cubic time when restricted
to quantifier-free entailment judgments. To show PSPACE-completeness,
we show that the entailment problem of FT$_\leq$ with existential
quantification is equivalent to the
inclusion problem of non-deterministic finite automata.
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