What is the difference between !! and nothing?
Double Negation in C++
Very strange error suddenly appeared running Biber I am on a Mac with MacTex 2020 and ran into a strange problem with Biber. Working on a document all was fine, I did not change the bibliography (says my git) and only fixing typos (not even adding cites) and suddenly latexmk stopped working after a Biber run, while that looked like  INFO - This is Biber 2.14 INFO - Logfile is 'doc.blg' INFO - Reading 'doc.bcf' INFO - Found 31 citekeys in bib section 0 INFO - Processing section 0 INFO - Globbing data source 'literature.bib' INFO - Globbed data source 'literature.bib' to literature.bib INFO - Looking for bibtex format file 'literature.bib' for section 0 INFO - LaTeX decoding ... INFO - Found BibTeX data source 'literature.bib'   so harmless in my view. Then with --debug I additionally get in the end.  INFO - LaTeX decoding ... warning: overriding existing definition of macro "jun" warning: overriding existing definition of macro "jul" warning: overriding existing definition of macro "sep" warning: overriding existing definition of macro "jan" [...] warning: overriding existing definition of macro "nov" INFO - Found BibTeX data source 'literature.bib' Unicode::UCD: failed to find unicore/version in /var/folders/hm/gt63cn0j2r7bwbzw7c1hww1r0000gn/T/par-626572676d616e6e/cache-2f8a664c3c933a44d096bcc8d018a5e92bced897/inc/lib /var/folders/hm/gt63cn0j2r7bwbzw7c1hww1r0000gn/T/par-626572676d616e6e/cache-2f8a664c3c933a44d096bcc8d018a5e92bced897/inc CODE(0x7fd026aeb5a8) CODE(0x7fd026aebb90) at /var/folders/hm/gt63cn0j2r7bwbzw7c1hww1r0000gn/T/par-626572676d616e6e/cache-2f8a664c3c933a44d096bcc8d018a5e92bced897/inc/lib/Biber/Input/file/bibtex.pm line 1011.   It seems unrelated to the library itself since deleting a few entries (going through deleting one half, then the other), the problem persists. It does not happen for an empty literature, but as soon as a first entry is added (just author/journal/title/year/volume/pages - no utf8-only chars) this error message appears.  what does this last error message mean?  How can I solve this? Returning from Dual-boot to just Windows I installed Ubuntu sometime back along side Vista, but later found that I rarely (if ever) use Ubuntu. How easy is it to return my computer to its initial state of having only one OS, and freeing up the hard-drive space used by the Ubuntu install? New top bar doesn't show hot threads across Stack Exchange sites I have noticed that the confidence interval for predicted values in an linear regression tends to be narrow around the mean of the predictor and fat around the minimum and maximum values of the predictor. This can be seen in plots of these 4 linear regressions:    I initially thought this was because most values of the predictors were concentrated around the mean of the predictor. However, I then noticed that the narrow middle of the confidence interval would occur even if many values of were concentrated around the extremes of the predictor, as in the bottom left linear regression, which lots of values of the predictor are concentrated around the minimum of the predictor.  is anyone able to explain why confidence intervals for the predicted values in an linear regression tend to be narrow in the middle and fat at the extremes? hashCode() for an array of objects for use in HashMap Cannot find Underground Jungle Temple ModuleNotFoundError: No module named 'apt_pkg' error I run below command on ubuntu 16.4 :  sudo add-apt-repository ppa:noobslab/apps  of below question:    and get below error:  Traceback (most recent call last):   File "/usr/bin/add-apt-repository", line 11, in &lt;module&gt;     from softwareproperties.SoftwareProperties import SoftwareProperties, shortcut_handler   File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 27, in &lt;module&gt;     import apt_pkg ModuleNotFoundError: No module named 'apt_pkg' Error in sys.excepthook: Traceback (most recent call last):   File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook     from apport.fileutils import likely_packaged, get_recent_crashes   File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in &lt;module&gt;     from apport.report import Report   File "/usr/lib/python3/dist-packages/apport/report.py", line 30, in &lt;module&gt;     import apport.fileutils   File "/usr/lib/python3/dist-packages/apport/fileutils.py", line 23, in &lt;module&gt;     from apport.packaging_impl import impl as packaging   File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 23, in &lt;module&gt;     import apt   File "/usr/lib/python3/dist-packages/apt/__init__.py", line 23, in &lt;module&gt;     import apt_pkg ModuleNotFoundError: No module named 'apt_pkg'  Original exception was: Traceback (most recent call last):   File "/usr/bin/add-apt-repository", line 11, in &lt;module&gt;     from softwareproperties.SoftwareProperties import SoftwareProperties, shortcut_handler   File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 27, in &lt;module&gt;     import apt_pkg ModuleNotFoundError: No module named 'apt_pkg'   Thanks for help What determines whether colors you can't see are visible or not? So, when someone is red-green colorblind, the colors appear the same to them, like this:   And if you're totally colorblind, then things presumably just appear like they would in a black-and-white movie.  However, this isn't how ultraviolet patterns seem to work. Compare how we see this flower to the version where ultraviolet is visible:   The UV pattern is completely invisible here. However, unlike with the red and green, this isn't because yellow and UV are colors that appear identical when you can't see UV. Look at these flowers:     This time the flowers are purple, but the UV pattern is still invisible. Why is that? Shouldn't the UV pattern still be apparent on at least one of the flowers, just in a different color? And on some other flowers, the UV does appear as a different color. So:    Why is the UV invisible only sometimes?  Does it have to do with the flower using iridescent structures to produce color, instead of a pigment? Can this happen with red and green, as well? Let $A,B \subseteq \mathbb{R}^d$ be non-empty sets. Define their distance to be $$ d(A,B) = \inf \{ \|x-y\| : x \in A, \; \; y \in B \} $$ For any $A,B$, I want to prove that $d(A,B) = d( \overline{A}, \overline{B} )$. My Attempt Put $\alpha = d(A,B)$. Therefore, $\alpha = \|x_0 - y_0 \|$ for some $x_0 \in A$ and some $y_0 \in B$. But, notice $x_0 \in \overline{A}$ and $y_0 \in \overline{B}$ by definition. hence $$ \| x_0 - y_0\| \geq \inf\{ \|x' - y'\| : x' \in \overline{A}, \; \; y' \in \overline{B} \} =  d( \overline{A}, \overline{B} )$$ $$\therefore d(A,B) \geq d( \overline{A}, \overline{B} )$$ I am stuck trying to show the other direction: $d(A,B) \leq d( \overline{A}, \overline{B} )$ Can someone help me? thanks a lot RedirectMatch and Query String Compare these two RedirectMatch's. The first doesn't work: RedirectMatch 302 ^/redirect\.php[?]page=(.+)$ http://somewhereelse.com/$1  Versus this, which will redirect to http://somewhereelse.com/?page=wherever: RedirectMatch 302 ^/redirect\.php([?]page=.+)?$ http://somewhereelse.com/$1  Does RedirectMatch only match the URI and not the query string? Apache's documentation is a bit vague in this area. What I'm trying to do is extract the page query parameter and redirect to another site using it. Is that possible with RedirectMatch or do I have to use RewriteCond + RewriteRule? How did Walter White poison that character? I've seen Breaking Bad complete series 2-3 times, but I never found out how Walter White was able to poison Brock.  I mean did he himself directly feed the Valley of the Lilly berries to Brock or did he let Saul do the work? I can't figure it out. I know from the  that it is not possible to introduce direct abbreviations like \newcommand{\bal}{\begin{align}} % doesn't work \newcommand{\eal}{\end{align}}   % doesn't work  or \newcommand{\bal}{\align}    % doesn't work \newcommand{\eal}{\endalign} % doesn't work  What is puzzling me is that \newenvironment{foo}{\align}{\endalign}  does work, at least if one is well-behaved enough to diligently type \begin{foo}...\end{foo}, since \foo...\endfoo won't work either. I'd really appreciate some help to shed some light on what is going on behind the scenes :-) Is there a good way to determine the value of magic items in 5e? In 5e, magic items aren't as common or as varied as in older editions, but that also means it is hard to place prices on them.  As DM, NPC's can't really sell a magic item because we don't know the prices for these items. While it is stated that they are rare and usually unpurchasable and unsellable, but I still have to have some basis on what to price them.  So is there a way to find buying and selling prices for magic items in 5e? How can I improve the performance of Minecraft? Extracting extension from filename in Python Is there a function to extract the extension from a filename? How can I prove $\sqrt{2} ^{\sqrt{2}}$ is irrational? Why am I getting 50V at a light when the switch is off? I am trying to put in a switch and a light it controls in a storage building and  is how I wired it.   Click for larger view  When I checked the wiring it registered 120V when the switch was on, and right around 50V when the switch was off.  Why would power still be getting to the light (I'm guessing there is a problem with the switch) and why would it only be 50V?  Oh, and where everything branches off at the beginning I have a junction box there where all of the wires are joined there. some \newcommand instructions not working Glassware with scratched/etched surfaces generate more bubbles (, ). Why these rough surfaces allow easier heterogeneous nucleation? If $A$ is a non-square matrix with orthonormal columns, what is $A^+$? If a matrix has orthonormal columns, they must be linearly independent, so $A^+ = (A^T A)^{−1} A^T$ . Also, the fact that its columns are orthonormal gives $A^T A = I$. Therefore,  $$A^+ = (A^T A)^{−1} A^T = (I)^{-1}A^T = A^T$$  Thus, $A^+ = A^T$. Am I correct? Thank you.