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Prints all routes for the default or a given router.

    $ mix phx.routes
    $ mix phx.routes MyApp.AnotherRouter

The default router is inflected from the application
name unless a configuration named `:namespace`
is set inside your application configuration. For example,
the configuration:

    config :my_app,
      namespace: My.App

will exhibit the routes for `My.App.Router` when this
task is invoked without arguments.

Umbrella projects do not have a default router and
therefore always expect a router to be given.
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