To use YASnippet in Emacs, add the following lines to your ~/.emacs:

  (require 'yasnippet)
  (yas/initialize)
  ;; for the default snippets:
  (yas/load-directory "%%PREFIX%%/share/yasnippet/snippets")

This way you will get a predefined, read-only behavior.

You may also use a different directory with your own snippets, and
copy any required snippets from the default directory.