Jade: Change active menu item in parent template

parent.jade

doctype 5

html
  block link
    -var selected = 'home'; //default

  -var menu = { 'home': '/home', 'blog': '/blog', 'contact': '/contact' };

  body
    nav
      ul
        each val, key in menu
          li
            if selected === key
              a.selected(href=val, title=key)= key
            else
              a(href=val, title=key)= key

child.jade

extends parent

block link
  -var selected = 'blog';