* is the only one in use, but if we encounter nested concatenations, we start a deque of
     * meta-iterators rather than letting the nesting get arbitrarily deep.  This keeps each
     * operation O(1).
     */

    private @Nullable Iterator<? extends Iterator<? extends T>> topMetaIterator;

    // Only becomes nonnull if we encounter nested concatenations.

    provided.
 *  Fix: Enforce that callers call `WebSocket.close()` on IO errors. Error
    handling in WebSockets is significantly improved.
 *  Fix: Don't use SPDY/3 style header concatenation for HTTP/2 request headers.
    This could have corrupted requests where multiple headers had the same name,
    as in cookies.
 *  Fix: Reject bad characters in the URL hostname. Previously characters like