_io_data
BPositionIO *_io_data;
_io_data->Seek(part->start,SEEK_SET);
if (component.SetToRFC822(_io_data,part->end - part->start) < B_OK)
_io_data->Seek(part->start,SEEK_SET);
if (piece->SetToRFC822(_io_data,part->end - part->start, parse_now) < B_OK)
_io_data = data;
_io_data->ReadAt(begin,buffer,length);