'Installs this `IOLoop` object as the singleton instance.
This is normally not necessary as `instance()` will create
an `IOLoop` on demand, but you may want to call `install` to use
a custom subclass of `IOLoop`.
When using an `IOLoop` subclass, `install` must be called prior
to creating any objects that implicitly create their own
`IOLoop` (e.g., :class:`tornado.httpclient.AsyncHTTPClient`).'

def install(self):

    assert (not IOLoop.initialized())
    IOLoop._instance = self
