identifiers.put(ident, method);
          }
        }
      }
    }
    return ImmutableList.copyOf(identifiers.values());
  }

  /** Global cache of classes to their flattened hierarchy of supertypes. */
  private static final LoadingCache<Class<?>, ImmutableSet<Class<?>>> flattenHierarchyCache =
      CacheBuilder.newBuilder()
          .weakKeys()
          .build(

 * map from keys to <i>nonempty</i> collections of values:
 *
 * <ul>
 *   <li>a → 1, 2
 *   <li>b → 3
 * </ul>
 *
 * ... or as a single "flattened" collection of key-value pairs:
 *
 * <ul>
 *   <li>a → 1
 *   <li>a → 2
 *   <li>b → 3
 * </ul>
 *
 * <p><b>Important:</b> although the first interpretation resembles how most multimaps are