public static IEnumerable<PhraseSequence> AggregateWords(this IEnumerable<PhraseSequence> phrases)
{
HashSet<PhraseSequenceElement> phraseSequenceSet = new HashSet<PhraseSequenceElement>(new PhraseElementComparerator());
foreach (var p in phrases) {
phraseSequenceSet.AddValidWords(p);
}
return phraseSequenceSet.Select(w => new PhraseSequence(w));
}