@karel looks like you did indeed discover additional information. Seems to moot my answer, so I have removed (deleted) the answer. Cheers and happy new year.
@user535733 It seems a shame to throw away the upvote that your answer received. Error messages of the type Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000𝐗𝐗00 have been solved by selecting a different kernel at boot time. I linked to this answer only so you could see the first two screenshots. I suggested that you edit your answer and then undelete it.
@user535733 I'm still researching the error message exitcode=0x00007f00. It seems like there may be more information to mine from this error code. See this link.
There a few not syncing: Attempted to kill init! exitcode=0x0000𝐗𝐗00 questions on Ask Ubuntu, most of them unanswered. My idea for 2020 is that someone should find the time to categorize these unanswered questions according to the exitcodes and try to answer some of them.
@karel Sounds good. Also, I'm interested in that Q&A (though I'm not sure I have any insight to contribute).
I presume the reason you prefer to use this room rather than AUGR (which is more common for Ubuntu-related discussion) is that you anticipate the conversation might become extremely long. Whether or not it does, that's of course fine.
If it comes to be interspersed with one or more separate unrelated conversations, then you might find it beneficial to use chat replies a bit more than you otherwise, so it's clear what relates to what.
Also, happy new year!
@Zanna I think I may have seen stuff like "¬∈" (or maybe "~∈") in old typewritten manuscripts. But meant as a ligature for "∉". Speaking of notation, I've sometimes used but not explicitly mentioned the parenthesized style "(∀x)" for quantifiers. This is especially useful when writing things like "(∀x ∈ S)" since it separates it from whatever formula follows it. "(∀x ∈ S) Fx" is shorthand for "∀x (x ∈ S → Fx)" and, relatedly, "(∃x ∈ S) Fx" is shorthand for "∃x (x ∈ S ∧ Fx)".
