cannot run example spec

[hraban]

When I run mdsh on the supplied spec files from the README I get errors.

Reproduce

$ nix run github:NixOS/nixpkgs/e57b3b16ad8758fd681511a078f35c416a8cc939#mdsh -- -i spec.clear.md

(I used the latest nixpkgs unstable version because this repo's master doesn't build for me locally)

What I expected

Successful exit, spec.processed.md and spec.clear.md to be equal

Observed

$ nix run nixpkgs/nixpkgs-unstable#mdsh  -- -i spec.clear.md
Using input=File("spec.clear.md") output=File("spec.clear.md") work_dir=Parent("/Users/user/tmp/mdsh") clean=false frozen=false
`> $ echo 'I am *markdown*'`
`> < ./samples/example.md`
`> yaml $ echo 'foo: true'`
`> yaml < ./samples/example.yml`
`> yaml foo: true`
`! $ echo 'foo=bar'`

thread 'main' panicked at src/main.rs:419:25:
WTF, not supported
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
$ echo $?
101

System

$ nix-info -m
 - system: `"aarch64-darwin"`
 - host os: `Darwin 24.6.0, macOS 15.6.1`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.28.3`
 - nixpkgs: `/nix/store/x06djk7jw02qb6y8fxd0mjgaksw2kd9n-source`