M.B. wrote an answer about Petzl Croll Chest Ascender on October 23, 2011
I would strongly recomend the use of a Maillon Rapide. Carabiners attached at the harness can easily be crossloaded as they become entangled in all the clutter of your attachment point. Rapieds are rated full strength in all directions and are the preferred attachment method. Also there the efficiency of an ascending system like the Frog system is impacted by how low the Croll is attached to the harness. Rapide's offer the the most compact attachment method for these systems.