What's the purpose of the 'in' keyword ?

[loloof64 via Digitalmars-d-learn]

Hello everyone,

I've just completed the language tour, and I am starting a 
tutorial in order to use Gtk binding.

But somewhere, they use the 'in' keyword for the constructor : 
(https://sites.google.com/site/gtkdtutorial/#chapter2 : in 
section 3 for Buttons and Callbacks, main.d snippet).

What's the purpose of this 'in' keyword ? I could not process a 
good Google request to get an answer.

Regards