code: @Override
    public void collect(SpanCollector collector) throws IOException {
      spans1.spans.collect(collector);
      spans2.spans.collect(collector);
    }
docs: /*
  (non-Javadoc)
  
  @see
  org.apache.lucene.search.spans.Spans#collect(org.apache.lucene.search.spans
  .SpanCollector)