The Code for insert-buffer

Here is the earlier code:

(defun insert-buffer (buffer)
  "Insert after point the contents of BUFFER.
Puts mark after the inserted text.
BUFFER may be a buffer or a buffer name."
  (interactive "*bInsert buffer: ")

  (or (bufferp buffer)
      (setq buffer (get-buffer buffer)))
  (let (start end newmark)
    (save-excursion
      (save-excursion
        (set-buffer buffer)
        (setq start (point-min) end (point-max)))

      (insert-buffer-substring buffer start end)
      (setq newmark (point)))
    (push-mark newmark)))

As with other function definitions, you can use a template to see an outline of the function:

(defun insert-buffer (buffer)
  "documentation..."
  (interactive "*bInsert buffer: ")
  body...)