I'm trying to make a filter to transform some features of org-mode to 
GitLab-markdown (not supported by Pandoc out of the box), in particular, 
the math blocks.

The filter should work when transforming to `markdown`, but instead of 
giving the markdown format for the math blocks (enclosed by `$$...$$`), it 
should write the blocks as 

    ```  math
    a + b = c
    ``` 

The preces I have now is 

In org-mode, the math blocks are simply the latex code:

```
\begin{equation}
a + b = c
\end{equation}
```

this is parsed as a pandoc AST `RawBlock` with format `latex`. I then 
remove the first (`\begin{equation}`) an last line (`\end{equation}`), and 
construct a pandoc `CodeBlock` with atrributes `{"math"}`, so the 
`CodeBlock` object displays in AST as

```
CodeBlock ("",["math"],[]) "a + b = c\n"
```

and then I let Pandoc create the markdown document, and the written result 
is 

    ``` {.math}
    a + b = c
    ``` 



**The question:**  
I want the bare `math`, not `{.math}` written, without the use of CLI 
options. 


I am aware that this can be done setting the Writer extension 
`fenced_code_attributes` to false (eg. `$pandoc -w 
markdown-fenced_code_attributes ...`), but I would much prefer this done 
inside the filter. 

Or is it possible to set the extensions inside the filter?


Here is my atempted lua-filter:

``` lua
function RawBlock(rb)
   if rb.format == "latex" then
      local text = rb.text
      split_text =  split(text, "[^\n]*")
      if split_text[1] == '\\begin{equation}'  and 
split_text[#split_text-1] == '\\end{equation}' then
         table.remove(split_text, #split_text-1)
         table.remove(split_text, 1)
         text = table.concat(split_text, "\n")
         local cb = pandoc.CodeBlock()
         cb.attr = {"",{"math"}}
         cb.text = text
         return cb
      end
   end
end
```


Kind regards, 

Angel




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