Non-hardcoded contents
From https://github.com/KOLANICH/kaitai_struct_formats/blob/CNTK/scientific/data_science/cntk.ksy
```yaml
types:
  begin_marker_hardcode:
    params:
      - id: name
        type: str
    seq:
      - id: end
        contents: ["B", 0]
      - id: marker
        contents: [name]
```

contents matches variable name instead of parameter.
It should match the string passed as a parameter. It would also be nice to specify string length in the parameter if the marker is fixed length. And string type (str or strz and terminator) too. And make the compiler cast types if it is possible or check if it is impossible to cast. Length mismatch should not be cast. str->strz and encodjngs should.