In this paper, we focus on parsing rare and
non-trivial constructions, in particular ellipsis. We report on several experiments in
enrichment of training data for this specific
construction, evaluated on five languages:
Czech, English, Finnish, Russian and Slovak.
These data enrichment methods draw upon
self-training and tri-training, combined with
a stratified sampling method mimicking the
structural complexity of the original treebank.
In addition, using these same methods, we
also demonstrate small improvements over the
CoNLL-17 parsing shared task winning system for four of the five languages, not only restricted to the elliptical constructions.