public function addCollection(ReferenceGeneratorCollection $collection)
    {
        // we need to remove all generators with the same names first because 
        // just replacing them
        // would not place the new generator at the end of the merged array
        foreach ($collection->all() as $generator) {
            $this->generators[] = $generator;
        }
        
        usort($this->generators,array($this,'compareGenerators'));
    }