* map.
   *
   * <p>The returned map isn't threadsafe or serializable, even if {@code unfiltered} is.
   *
   * <p>Many of the filtered map's methods, such as {@code size()}, iterate across every key/value
   * mapping in the underlying map and determine which satisfy the filter. When a live view is
   * <i>not</i> needed, it may be faster to copy the filtered map and use the copy.
   *

   *       Executor)} instead, with a function that returns the next value directly.
   *   <li>Call {@link DeferredCloser#eventuallyClose(Object, Executor) closer.eventuallyClose()}
   *       for every closeable object this step creates in order to capture it for later closing.
   *   <li>Return a {@code ClosingFuture}. To turn a {@link ListenableFuture} into a {@code
   *       ClosingFuture} call {@link #from(ListenableFuture)}.