public Stream ExtractRevision(String field) {
FindSignatureNames();
field = GetTranslatedFieldName(field);
if (!sigNames.ContainsKey(field))
return null;
int length = ((int[])sigNames[field])[0];
RandomAccessFileOrArray raf = reader.SafeFile;
raf.ReOpen();
raf.Seek(0);
return new RevisionStream(raf, length);
}