code: public int getDocumentNumber(ForeignSegmentDocId docId) {
          OffsetReader r = (OffsetReader) readersByCreationTick.get(
                  new Long(docId.getCreationTick()));
          if (r != null && !r.reader.isDeleted(docId.getDocNumber())) {
              return r.offset + docId.getDocNumber();
          }
          return -1;
      }
docs: {@inheritDoc}