CDRs are in ASN.1 form. ASN.1 definition was in some doc I forgot where. Hand-coding all ASN.1 stuff is ridiculous. So.
syntax.py is using pyparsing to parse syntax.txt (which is a syntax). Then
you make it into defs.py by adding imports line on top.
Next, use decode.py on binary CDR file from the switch. The rest is DIY.