mounted_helpers are a bit similar to url_helpers. They're automatically
included in controllers for Rails.application and each of mounted
Engines. Mounted helper allows to call url_for and named helpers for
given application.

Given Blog::Engine mounted as blog_engine, there are 2 helpers defined:
app and blog_engine. You can call routes for app and engine using those
helpers:

app.root_url
app.url_for(:controller => "foo")
blog_engine.posts_path
blog_engine.url_for(@post)