Default theme implementation to navigate books.
Presented under nodes that are a part of book outlines.
- $tree: The immediate children of the current node rendered as an unordered
- $current_depth: Depth of the current node within the book outline. Provided
- $prev_url: URL to the previous node.
- $prev_title: Title of the previous node.
- $parent_url: URL to the parent node.
- $parent_title: Title of the parent node. Not printed by default. Provided
as an option.
- $next_url: URL to the next node.
- $next_title: Title of the next node.
- $has_links: Flags TRUE whenever the previous, parent or next data has a
- $book_id: The book ID of the current outline being viewed. Same as the node
ID containing the entire outline. Provided for context.
- $book_url: The book/node URL of the current outline being viewed. Provided
as an option. Not used by default.
- $book_title: The book/node title of the current outline being viewed.
Provided as an option. Not used by default.