// Technically this violates the one-synced-object-per-thread
// principle (syncing on data and on an individual). Don't think
// it's a problem.
sclass SyncListMultiMap<A, B> extends MultiMap<A,B> {

  *() {}
  *(bool useTreeMap) { super(useTreeMap); }
  *(Map<A, L<B>> *data) {}
  
  L<B> _makeEmptyList() {
    ret new SynchronizedArrayList;
  }
}