鉴于此/etc/apt/sources.list文件：
deb http://us.archive.ubuntu.com/ubuntu/ cosmic main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ cosmic main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu/ disco main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ disco main restricted universe multiverse
这个/etc/apt/preferences文件：
Package: *
Pin: release a=cosmic
Pin-Priority: 900
Package: *
Pin: release a=disco
Pin-Priority: -10
正如我所期望的那样，运行apt install some-package将从中安装版本。cosmic
但是，运行apt source some-package将从disco.
我怎样才能使apt source服从引脚优先级？