markdown to markdown, stripping Yaml comments and keeping 3-backticks code blocks intact

[bapt a <auguieba-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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Hi,

I'm playing with an idea for quarto, where one could comment out portions 
of the input document including text and code chunks. This workaround would 
make use of the trick that 1) Yaml blocks can appear anywhere in a pandoc 
document, and 2) commented out (#) lines in Yaml are stripped by pandoc (?).

I'd like to run a pre-processing step* on the input document to return 
essentially the same input, but getting rid off those commented out Yaml 
lines.

Consider the following input file,

``````
---
title: Comment
---

## Section

This is a simple placeholder for the manuscript's main document [@knuth84].


```{r}
x <- 1 + 1
x
```

Note pi is `r pi`.

---
#  You can embed code like this:
# 
# ```{r}
# 1 + 1
# ```
# but beware, is `r .NotYetImplemented()` still executed?
---

More text.

``````
The commented out portion should be removed before quarto calls the 
computational engine, otherwise the code still gets executed.

I've tried various flavours of markdown, such as

> quarto pandoc -t noop.lua  -f markdown  index.qmd

where noop.lua is a dummy Writer doing nothing,

```
function Writer (doc, opts)
  local filter = {  }
  return pandoc.write(doc:walk(filter), 'markdown', opts)
end
````

but I'm not finding the right combination of options, if there is one. Some 
markdown flavours seem to keep the Yaml comments (not sure why), others 
reformat the 3-backticks code block. Is this "identity" Writer going to be 
difficult to achieve?

Many thanks,

baptiste

* Note that code is currently detected via regex in quarto, so it gets 
executed even inside comments (hence the need of a pre-processor to strip 
it entirely from the document).
See https://github.com/quarto-dev/quarto-cli/discussions/3330 for more 
context

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