static <A> AutoCloseable tempSyncAdd(Cl<A> l, A a) {
  if (l == null) null;
  if (!syncAdd(l, a)) null;
  ret new tempSyncAdd_undo(l, a);
}

srecord noeq tempSyncAdd_undo<A>(Cl<A> l, A a) is AutoCloseable {
  close { syncRemove(l, a); }
}