this is my controller in CI

class Welcome extends Controller {

 function Welcome()
 {
  parent::Controller(); 
  }

 function index()
 {

 }
 function bil($model='')
 { }

I want to do a rewrite so that

http://example.com/index.php/welcome/bil/model

becomes

http://example.com/model

in my htaccess I have

RewriteBase /
RewriteCond $1 !^(index\.php|images|robots\.txt)
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /index.php/welcome/$1 [L]
#RewriteRule ^(.*)$ /index.php/welcome/bil/$1 [L]

I thought it should be as easy as removing the /index.php/welcome/ part but when I uncomment the last line it get 500 internal server error