[Vala] How to use Posix namespace

[Andrey -Zox- Kucherov <zox nsk raid ru>]

Hello,
Sorry for newbie question... but how can I use Posix namespace in my
project?

When I type
---
using Posix;
---
The compiler returns an error
---
The namespace name `Posix' could not be found
---
However I was able to find the posix.vapi file
in /usr/share/vala/vapi/. 
How do I use it?