Function wp() in wp-blog-header.php

Hello. I have a little question for wordpress developers. Or maybe someone else could help me?

I’m trying to understand logic of WordPress source code. But there is one problem in wp-blog-header.php file. How is it possible that calling function wp() is not causing an error? This function is defined in file ‘./wp-includes/functions.php’ and this file was not included before.

P.S: sorry for my English. I’m still learning it.