punctuation) are more likely to appear in multiple orientations with a given phrase, and therefore provide poor evidence of reordering.
we (a) collect the longest contextual phrases (which also appear in the phrase table) for reordering feature estimation, and (b) prune the set of sentences so that we only keep a small set of least frequent contextual phrases (this has the effect of dropping many function words and punctuation marks and and relying more heavily on multi-word content phrases to estimate the reordering).