sclass CloseablesHolder implements AutoCloseable {
  // TODO: combine with set for speed? what about multiple additions of the same element though
  new L<AutoCloseable> closeables;
  
  <A extends AutoCloseable> A add(A a) {
    if (a != null)
      closeables.add(a);
    ret a;
  }
  
  // remove from list, don't close
  <A extends AutoCloseable> A remove(A a) {
    closeables.remove(a);
    ret a;
  }
  
  // synonym of remove
  <A extends AutoCloseable> A takeOut(A a) {
    ret remove(a);
  }
  
  <A extends AutoCloseable> A close(A a) {
    if (a == null) ret a;
    closeables.remove(a);
    main close(a);
    ret a;
  }
  
  public void close {
    while (nempty(closeables))
      main close(popLast(closeables));
  }
}