How do you generate a TOC (Table Of Contents) in markdown-mode?

Like this:

(defun px/markdown-toc () "Extract level 2 and 3 headings from the current Markdown buffer. The generated and indented TOC will be inserted at point." (interactive) (let (toc-list markdown-toc) (save-excursion (goto-char (point-min)) (while (re-search-forward "^\\(##+\\)\\s-+\\(.*\\)" nil t) (let* ((level (length (match-string 1))) (heading-text (match-string 2)) (heading-id (downcase (replace-regexp-in-string "[[:space:]]+" "-" heading-text)))) (push (cons level (cons heading-text heading-id)) toc-list)))) (setq toc-list (reverse toc-list)) (dolist (item toc-list) (let* ((level (car item)) (heading-text (cadr item)) (heading-id (cddr item)) (indentation (make-string (* 2 (1- level)) ?\ )) (line (format "- [%s](#%s)\n" heading-text heading-id))) (setq markdown-toc (concat markdown-toc (concat indentation line))))) (insert markdown-toc)))