check with dxf-check:

• AcDbViewportTable on out_dxf blank_2000.dwg:
  Object improperly read: AcDbViewportTable (8)
  the dxf is written as 2 VPORT 70 2 (num_entries), but with NULL entries it must be 1.
  needs a dxf TABLE_WRITE_FIXUP_NUMENTRIES
• Object improperly read: AcDbHatch
• CELLSTYLEMAP cannot be encoded, because the TABLESTYLE is missing, because unstable.
  When we add TABLESTYLE to the DXF (which is safe to do <r2010), we still get
  Object of class AcDbCellStyleMap can't be cast to AcDbEntity. (2000/Circle)
• duplicate VPORT_CONTROL.entries (sample_2000)
• Object of class AcDbViewportTableRecord can't be cast to AcDbEntity (sample_2000)
• Object of class AcDbRegAppTableRecord can't be cast to AcDbEntity (2000/A)
• Object of class AcDbVXTable can't be cast to AcDbDictionary (citroen_from_cadforum.cz)
• Object was permanently erased: (7C) (2000/Ceco.NET-People-Men-Sm-471)
• Wrong blockname output, leading to missing INSERT.block_header with anonymous blocks (e.g. *X2 => *X)

more issues:

• CRC does not match (example_2000)
• x LAYER.flag0 0x3f0 => 0 (sample_2000). with json-check it is => 0x3e0
• x LTYPE->size off by one in 1 of 3. Not TV-ZERO