docs: private KeyCache<T_IdKeyPair, T_IdKey, T_PreKey, T_SigPreKey, T_Sess> getCache(OmemoDevice device) {
          KeyCache<T_IdKeyPair, T_IdKey, T_PreKey, T_SigPreKey, T_Sess> cache = caches.get(device);
          if (cache == null) {
              cache = new KeyCache<>();
              caches.put(device, cache);
          }
          return cache;
      }