我已经添加了 cassou ppa，但是当我尝试安装 emacs24 或 emacs-snapshot 时，apt-get 似乎找不到正确的包：
$ sudo add-apt-repository ppa:cassou/emacs
...
$ sudo apt-get update
...
$ sudo apt-get install emacs24 emacs24-el emacs24-common-non-dfsg
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package emacs24
E: Unable to locate package emacs24-el
E: Unable to locate package emacs24-common-non-dfsg
$ sudo apt-get install emacs-snapshot
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package emacs-snapshot is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'emacs-snapshot' has no installation candidate