Jade templating, variable scope in includes

I am using Jade (without Express, just for static HTML templating) - I understood includes to work as partials meaning scope is not an issue but this doesn't seem to be the case and I can find no reference to this use-case

master.jade

!!! 5
html
  block vars
  - var slug= 'home'
  head
    block pagetitle
      title Static HTML
    link(rel='stylesheet', href='css/styles.css')
  body(class= slug)
    .wrapper
      include includes/header

includes/header.jade

.header 
  ul
    li(class= slug)

I have tried syntax variants including #{slug} and always get error "slug is not defined" within the includes/header.jade file - can I do this?

EDIT: So the answer as given by Dave Weldon in the comments below is that the variable is available when included in master.jade but my build command compiled all jade files including the includes on their own, at which point the variable is of course not defined.

You could accomplish this with a mixin like so:

master.jade

include includes/header

!!!
html
  block vars
  - var slug= 'home'
  head
    block pagetitle
      title Static HTML
    link(rel='stylesheet', href='css/styles.css')
  body(class= slug)
    .wrapper
      mixin header(slug)

includes/header.jade

mixin header(klass)
  .header
    ul
      li(class= klass)

When compiled:

<!DOCTYPE html>
<html>
  <head>
    <title>Static HTML</title>
    <link rel="stylesheet" href="css/styles.css">
  </head>
  <body class="home">
    <div class="wrapper">
      <div class="header">
        <ul>
          <li class="home"></li>
        </ul>
      </div>
    </div>
  </body>
</html>