This generator function is designed to be used inside a "foreach" loop.
	 *
	 * Example:
	 *
	 *     $html = '<em><strong><a>We are here';
	 *     foreach ( $stack->walk_up() as $node ) {
	 *         echo "{$node->node_name} -> ";
	 *     }
	 *     > A -> STRONG -> EM ->
	 *
	 * To start with the first added element and walk towards the bottom,
	 * see WP_HTML_Open_Elements::walk_down().
	 *
	 * @since 6.4.0
	 * @since 6.5.0 Accepts $above_this_node to start traversal above a given node, if it exists.
	 *
	 * @param WP_HTML_Token|null $above_this_node Optional. Start traversing above this node,
	 *                                            if provided and if the node exists.
	 */
	public function walk_up( ?WP_HTML_Token $above_this_node = null ) {
		$has_found_node = null === $above_this_node;

		for ( $i = count( $this->stack ) - 1; $i >= 0; $i-- ) {
			$node = $this->stack[ $i ];

			if ( ! $has_found_node ) {
				$has_found_node = $node === $above_this_node;
				continue;
			}

			yield $node;
		}
	}

	/*
	 * Internal helpers.
	 */

	/**
	 * Updates internal flags after adding an element.
	 *
	 * Certain conditions (such as "has_p_in_button_scope") are maintained here as
	 * flags that are only modified when adding and removing elements. This allows
	 * the HTML Processor to quickly check for these conditions instead of iterating
	 * over the open stack elements upon each new tag it encounters. These flags,
	 * however, need to be maintained as items are added and removed from the stack.
	 *
	 * @since 6.4.0
	 *
	 * @param WP_HTML_Token $item Element that was added to the stack of open elements.
	 */
	public function after_element_push( WP_HTML_Token $item ): void {
		$namespaced_name = 'html' === $item->namespace
			? $item->node_name
			: "{$item->namespace} {$item->node_name}";

		/*
		 * When adding support for new elements, expand this switch to trap
		 * cases where the precalculated value needs to change.
		 */
		switch ( $namespaced_name ) {
			case 'APPLET':
			case 'BUTTON':
			case 'CAPTION':
			case 'HTML':
			case 'TABLE':
			case 'TD':
			case 'TH':
			case 'MARQUEE':
			case 'OBJECT':
			case 'TEMPLATE':
			case 'math MI':
			case 'math MO':
			case 'math MN':
			case 'math MS':
			case 'math MTEXT':
			case 'math ANNOTATION-XML':
			case 'svg FOREIGNOBJECT':
			case 'svg DESC':
			case 'svg TITLE':
				$this->has_p_in_button_scope = false;
				break;

			case 'P':
				$this->has_p_in_button_scope = true;
				break;
		}

		if ( null !== $this->push_handler ) {
			( $this->push_handler )( $item );
		}
	}

	/**
	 * Updates internal flags after removing an element.
	 *
	 * Certain conditions (such as "has_p_in_button_scope") are maintained here as
	 * flags that are only modified when adding and removing elements. This allows
	 * the HTML Processor to quickly check for these conditions instead of iterating
	 * over the open stack elements upon each new tag it encounters. These flags,
	 * however, need to be maintained as items are added and removed from the stack.
	 *
	 * @since 6.4.0
	 *
	 * @param WP_HTML_Token $item Element that was removed from the stack of open elements.
	 */
	public function after_element_pop( WP_HTML_Token $item ): void {
		/*
		 * When adding support for new elements, expand this switch to trap
		 * cases where the precalculated value needs to change.
		 */
		switch ( $item->node_name ) {
			case 'APPLET':
			case 'BUTTON':
			case 'CAPTION':
			case 'HTML':
			case 'P':
			case 'TABLE':
			case 'TD':
			case 'TH':
			case 'MARQUEE':
			case 'OBJECT':
			case 'TEMPLATE':
			case 'math MI':
			case 'math MO':
			case 'math MN':
			case 'math MS':
			case 'math MTEXT':
			case 'math ANNOTATION-XML':
			case 'svg FOREIGNOBJECT':
			case 'svg DESC':
			case 'svg TITLE':
				$this->has_p_in_button_scope = $this->has_element_in_button_scope( 'P' );
				break;
		}

		if ( null !== $this->pop_handler ) {
			( $this->pop_handler )( $item );
		}
	}

	/**
	 * Clear the stack back to a table context.
	 *
	 * > When the steps above require the UA to clear the stack back to a table context, it means
	 * > that the UA must, while the current node is not a table, template, or html element, pop
	 * > elements from the stack of open elements.
	 *
	 * @see https://html.spec.whatwg.org/multipage/parsing.html#clear-the-stack-back-to-a-table-context
	 *
	 * @since 6.7.0
	 */
	public function clear_to_table_context(): void {
		foreach ( $this->walk_up() as $item ) {
			if (
				'TABLE' === $item->node_name ||
				'TEMPLATE' === $item->node_name ||
				'HTML' === $item->node_name
			) {
				break;
			}
			$this->pop();
		}
	}

	/**
	 * Clear the stack back to a table body context.
	 *
	 * > When the steps above require the UA to clear the stack back to a table body context, it
	 * > means that the UA must, while the current node is not a tbody, tfoot, thead, template, or
	 * > html element, pop elements from the stack of open elements.
	 *
	 * @see https://html.spec.whatwg.org/multipage/parsing.html#clear-the-stack-back-to-a-table-body-context
	 *
	 * @since 6.7.0
	 */
	public function clear_to_table_body_context(): void {
		foreach ( $this->walk_up() as $item ) {
			if (
				'TBODY' === $item->node_name ||
				'TFOOT' === $item->node_name ||
				'THEAD' === $item->node_name ||
				'TEMPLATE' === $item->node_name ||
				'HTML' === $item->node_name
			) {
				break;
			}
			$this->pop();
		}
	}

	/**
	 * Clear the stack back to a table row context.
	 *
	 * > When the steps above require the UA to clear the stack back to a table row context, it
	 * > means that the UA must, while the current node is not a tr, template, or html element, pop
	 * > elements from the stack of open elements.
	 *
	 * @see https://html.spec.whatwg.org/multipage/parsing.html#clear-the-stack-back-to-a-table-row-context
	 *
	 * @since 6.7.0
	 */
	public function clear_to_table_row_context(): void {
		foreach ( $this->walk_up() as $item ) {
			if (
				'TR' === $item->node_name ||
				'TEMPLATE' === $item->node_name ||
				'HTML' === $item->node_name
			) {
				break;
			}
			$this->pop();
		}
	}

	/**
	 * Wakeup magic method.
	 *
	 * @since 6.6.0
	 */
	public function __wakeup() {
		throw new \LogicException( __CLASS__ . ' should never be unserialized' );
	}
}
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