Setting ac-sources

[Florian Lindner]

Hello,

I set my ac-sources of auto-complete like that

(add-hook 'c-mode-common-hook (lambda()
   (setq ac-sources '(ac-source-semantic ac-source-yasnippet))))


Now I open a cpp file and look at the variable ac-sources which has the 
value:

(ac-source-yasnippet ac-source-gtags ac-source-semantic ac-source-yasnippet)

Why not just (ac-source-semantic ac-source-yasnippet)? It seems the setq 
appended instead of setting the list?

I'm a elisp newbie and hope for an explanation.

Thanks,
Florian