Running Ubuntu 20.04 and I think I've hit a regression with some third party software.  I need to roll back my installation to a specific point in time, e.g. 3 months ago.  I'd prefer to re-deploy the image with exactly the package versions that were available on a certain date, but I can't see how to specify that. The manifest files are useful (e.g. ) but they only seem to cover packages that are part of the installer, not the additional package selections that might be downloaded from XX.archive.ubuntu.com. Does anyone perhaps maintain a package list indexed by date? Thanks!
Debian users have a kind of "wayback machine":  , which is really great.  There are regressions sometimes, and if I don't have local deb package with old version, I'm getting sad sometimes. Having a wayback machine like this is very helpful.  I can't find analogue of it for Ubuntu. Have I missed it?
Underscores in custom fields While trying to perform a WP_Query using meta_query(), I had some trouble because the custom fields created by a plugin had _ in the beginning of its name.   Although I noticed it was stored this way in the database, I had no idea why it was there and thought I had to reference it without the underscore (for the record, that is wrong).  Why do some custom fields start with underscores and others don't? What are the underscores used for? Are they mandatory in some circumstances? Can't Prove formula from Sakurai's Modern QM @ Perturbation Theory I am studying Perturbation Theory from J.J. Sakurai's textbook Modern Quantum Mechanics. I am having trouble proving formulas on page 299 (5.2.5) and (5.2.6) from the previous ones [mainly (5.2.4)]. Can anyone help?  This is a problem of working with perturbation theory when the non-perturbed state has degeneracy. To elaborate I do not get why $(E-H_0-\lambda P_1VP_1)$ is not singular.  More importantly is the equation below correct and if so why:  $$(E-\lambda P_1VP_1)|l \rangle = E_D^{(0)}|l \rangle $$  That is the core of my problem. If the above equation - in the framework of Sakurai's statement of the problem - is correct I can work out all the rest on my own. However I do not get why this equation would hold true. How can I add a right-click desktop menu entry to create a text file to Gnome 3 in Ubuntu 16.04.02? Display sheet name in Google Spreadsheets Is there any way to display a sheet name as the result of a function in a Google Spreadsheets (exactly like , except in Google Sheets)? Where is the denoiser? Learning Python programming with generic GIS goals in mind? In your opinion, what is the best book/site to learn Python with GIS goals in mind?  By "best", I mean:   not very long (book) easy to understand (book/site) good practical examples (book/site)   For answers specific to learning how to use the ArcPy site-package for Python to customize ArcGIS for Desktop there is a separate Q&amp;A: In SQL Server 2005, are there any disadvantages to making all character fields nvarchar(MAX) rather than specifying a length explicitly, e.g. nvarchar(255)? (Apart from the obvious one that you aren't able to limit the field length at the database level) If $\psi h$ is in $L^2$ for all $h\in L^2$, must $\psi$ be essentially bounded? If $\psi:[a,b]\to\mathbb C$ is a (measurable) function such that $\psi h$ is in $L^2[a,b]$ for all $h\in L^2[a,b]$, then must $\psi$ be essentially bounded?  The converse direction is clear: If $\psi$ is essentially bounded, then $\|\psi h\|_2\leq \|\psi\|_\infty\|h\|_2$ for all $h\in L^2$, and thus the multiplication operator $M_\psi:h\mapsto \psi h$ is defined and bounded.  The question here is whether, without prior assumptions on $\psi$, simply having $M_\psi(L^2)\subseteq L^2$ implies that $\psi$ is bounded. How do I sort a dictionary by value? What is the "correct" way of embedding text into math mode? Lets say I have something like:   \[ Let x = number of cats \]   (This is a contrived example, but pretend we are actually inside a long align environment block...) "Let" and "number of cats" need to be written like normal text, while x= needs to be math type text.  I've seen this:  \[ \mbox{Let } x = \mbox{ number of cats} \]   and this:  \[ \textrm{Let } x = \textrm{ number of cats} \]   They seem to produce equivalent output, but I'm not sure which is the "correct" one. Also, they are not ideal in that the extra space needs to be placed inside the text mode block, otherwise it is ignored as math mode and butts the text right up against the math.  Ideally there would be some command which allowed embedding text into an equation, which correctly inserted leading and trailing space around the block as required without adding space inside the text block.  How does one do that "correctly"? Why is my Nikon 50mm f/1.8 giving me an fEE error? I have a AF Nikkor 50mm f/1.8. When I have the aperture setting on the lens to 22 I can take photos. But when I change the dial to anything other than 22(lowest f/stop) then I get a flashing fEE message and r09 where the number of exposure left is.  I don't have the manual for this lens but am just wondering if anyone knows what the issue is here.  My camera is a Nikon D70S. I asked a question on Stack Overflow: . I want to award my bounty to  by user  However, my Stack Overflow account is suspended recently for 7 days, and the suspension still has 5 more days to end.And I know I can't vote, ask, answer or comment as pointed on  by  with the sentence below.  You will be unable to vote, ask, answer, or comment.  I couldn't find anything about not being able to accept an answer or award bounty so I tried to accept the mentioned answer above and award my bounty to it and I got a  Suspended users cannot vote  rejection which can be seen below with a screenshot.  I know this  If you do not award your bounty within 7 days, the highest voted answer created after the bounty started with at least 2 upvotes will be awarded half the bounty amount.  from . None of the answer given to my question is upvoted and the answer I mentioned is not the oldest one after the bounty started so it won't be awarded by community. What can I do? (except being a good guy and not get suspended :)) Canonical URL for a home page and trailing slashes Are collective nouns always plural, or are certain ones singular? I'd say Microsoft have a way of bending the rules and I know that McLaren have won the championship. While this sounds strange, I believe it is correct English (sorry, I'm not native).   But when it's a small company, would you still use it this way? Is a company always plural, or are small companies singular? I.e., would you say Bakery Johnson makes fine bread or Bakery Johnson make fine bread? Is it My book seller, Woody's, have moved or is it has moved? Imported CSV data is not displayed in QGIS I watched a simple video tutorial on how to accomplish it, but nothing shows up on my project when I add the layer. Even if I try zoom to layer, the screen just shows blue. I'm not sure if it has something to do with my Excel, computer, or program. I really need help as I am new to QGIS. (I just posted this, but it won't let me comment to people under that question any more) I have a layer of google hybrid, but when I do add the new layer it is at the top and checked transparency and everything. Its just not accepting the data I entered into the excel spreadsheet is my guess to what the issue is.  I have the spreadsheet with the name, longitude, latitude. Save as CSV In QGIS go to Layer &gt; Add layer &gt; Add Delimited Text Layer Upload the CSV Excel file Add  Nothing shows even tho there is a line in the layers (bottom left) showing that &quot;something&quot; is there but if I choose &quot;zoom to layer&quot; it changes to a blue screen I have a Base layer Google Maps hybrid, and an area of interest on Georgia What is the Total Reputation field on Area51? What does the total reputation field on Area51 represent?  It's obviously not the total of the user's reputation on the various sites, as that is the number listed as user reputation. My office is a converted garage about 30 feet from my house.   I have a coaxial cable already running from my house to the office.   I want to attach a Ethernet cable from my DSL modem, in my office, to a computer in my house.   Since I already have the coaxial cable installed, I bought two coax to Ethernet converters (NVA-P-860-BP78 Video Balun), but when I hooked them up I am getting no signal. It was my understanding that an Ethernet signal could travel over a coaxial cable.   Is that wrong? Any ideas why this isn't working? Abelian groups of order n. Is there a number $n$ such that there are exactly 1 million abelian groups of order $n$?   Can anyone please explain. I would yes because numbers are infinitive, and so any number n can be expressed as a direct product of cyclic groups of order n.  Can anyone please help  me understand.  Thank you. (java) ObjectInputStream deserializing wrong version of object How to remove my passport from a blacklist in Indian Immigration?