code: @Override
    public MtasSpanWeight createWeight(IndexSearcher searcher,
        boolean needsScores, float boost) throws IOException {
      SpanWeight subWeight = clause.createWeight(searcher, false, boost);
      return new SpanUniquePositionWeight(subWeight, searcher,
          needsScores ? getTermContexts(subWeight) : null, boost);
    }
docs: /*
  (non-Javadoc)
  
  @see
  org.apache.lucene.search.spans.SpanQuery#createWeight(org.apache.lucene.
  search.IndexSearcher, boolean)