I sometimes manually install a Debian package, whilst trying to figure out dependencies for something else, and then discover I should have installed a different "parent" package that will pull it in.
In the past I have left this situation as-is or, if I'm feeling particularly finicky removing the manually installed package and then installing the parent which re-installs it as an automatic dependency.
Anyway, there's a proper way to set a package as automatic (so
$ apt-get autoremove will remove it if it's no longer required) and I finally looked it up. Here it is in all it's simplistic glory:
This Blog post is almost entirely an aide-memoir to myself!