code: public InputStream extractRevision(String field) throws IOException {
          getSignatureNames();
          field = getTranslatedFieldName(field);
          if (!sigNames.containsKey(field))
              return null;
          int length = ((int[])sigNames.get(field))[0];
          RandomAccessFileOrArray raf = reader.getSafeFile();
          raf.reOpen();
          raf.seek(0);
          return new RevisionStream(raf, length);
      }
docs: Extracts a revision from the document.
  
  @param field the signature field name
  @return an <CODE>InputStream</CODE> covering the revision. Returns <CODE>null</CODE> if
  it's not a signature field
  @throws IOException on error