How to return user name from vibed session?

[Suliman via Digitalmars-d-learn]

Vibed have method get for user session 
http://vibed.org/api/vibe.http.session/SessionStore

I set user name for session like this:
req.session.set("username", "admin");
But I can't understand how to get user name from it:

abstract std.variant.VariantN!(20) get(
  string id,
  string name,
  lazy std.variant.VariantN!(20) defaultVal
);

What does every string here mean? Could anybody to show example 
of usage?