This file: http://user.it.uu.se/~embe8573/tut/purge.txt

         .-----------------------------.
         :    purge mono manually      :
         :    ···················      :
         :      by Emanuel Berg        :
         :   suggest improvements at   :
         : moasenwood AT zoho DOT eu   :
         :     compiled 2018-12-15     :
         : with help from alt.os.linux :
         '-----------------------------'

Note: Here '#' denotes a comment, not a superuser command.

# get all the libmono stuff
# the sed comic-book insult is to remove color output
$ apt list --installed | grep libmono | sed -r "s/\x1B\[([0-9]{1,2}(;[0-9]{1,2})?)?[m|K]//g" > mono

# open the file 'mono' in Emacs
(replace-regexp "/.*" "")
(replace-regexp "\n" " ")
# now we have a single-line, spaced list

$ sudo dpkg -P --force-all [the-list] cli-common mono-runtime
$ sudo dpkg -P -a

# note: this step I had to repeat several times
$ sudo apt-get update
$ sudo apt-get upgrade > still-mono

# open the file 'still-mono' in Emacs
# kill manually these lines:

    Reading package lists...
    Building dependency tree...
    Reading state information...
    You might want to run 'apt-get -f install' to correct these.
    The following packages have unmet dependencies:

# then, to get a list:
(replace-regexp " :.*" "")
(replace-regexp "Depends:.*" "")
(replace-regexp "Recommends:.*" "")
(replace-regexp "^ " "")
(replace-regexp "^[[:space:]].*$" "")
(replace-regexp "^[[:space:]]*$" "")
(replace-regexp "^\n" "")
(replace-regexp "\n" " ")

$ sudo dpkg -P --force all [the-list]
$ sudo dpkg -P -a

$ sudo apt-get update
$ sudo apt-get upgrade
# repeat all this until 'sudo apt-get upgrade' no longer complains

S sudo apt-get autoremove

# one final check
$ sudo apt-get update
$ sudo apt-get upgrade