This extension rewrites 2 blocks Mage_Catalog_Block_Layer_View and Mage_CatalogSearch_Block_Layer because magento does not offer any valid events to inject additinoal filters.
Or maybe I missed them. If someone spots one, please spread the word.
The blocks that override the ones mentioned above, don't do much. They just call the parent methods and dispatch an event that is observed by the same extension.
This should make it easier to fix conflicts. Conflicts
Since this extension rewrites 2 blocks, there is a high chance that it will conflict with other extensions that affect the layered navigation. And there are a lot of them out there.
To solve such a conflict, you need to edit the blocks Easylife_StockFilter_Block_Catalog_Layer_View and Easylife_StockFilter_Block_Catalogsearch_Layer and make them extend the blocks from the conflicting extension.
Then edit app/etc/modules/Easylife_StockFilter.xml and add the name of the conflicting extension inside the <depends> tag.
This way, the Stock filter extension is loaded after the other extension that changes the layered navigation, the methods rewritten in the extension blocks do the same thing as their parent blocks to and still dispatch the events needed for the additional filter.