Including a pug filter which name is in a variable

Question

In the Pug Language reference there is an easy example on how to use the markdown filter on a .md file and include it.

include:markdown-it myfile.md

However, I can't get to work reading from a file which name is in a variable. I expected this syntax to work:

include:markdown-it ${article}

Being var article = 'myfile.md'. But the code crash saying it can't find file '${article}' in my views folder.

What is the correct way of doing it? Or is it just imposible?

Regards.

EDIT: As suggested here, it's not possible to do what I want. The solution I found was rendering the markdown file first,

function loadmd(md) {
    var fs = require('fs');
    var markdown = require('markdown-it')();
    var mdfile = fs.readFileSync(__dirname + '/mds/' + md + '.md');
    return markdown.render(mdfile.toString());
}

and then pass it to the pug renderer as a variable:

res.render('art', {md: loadmd(req.params.article)}, function renderDone(err, html) { ... }

Template:

doctype html
html(lang=es)
head
    include htmlhead
    title !{pagetitle}
body
    include pageheader
    include pagemenu
    section
        article !{md}
    footer
        include pagefooter