8.11.1 Top new questions this week

8.11.1.1 Is there a LaTeX 3 transition guide for users (not gurus)?
8.11.1.2 LaTeX mistake doesn't throw error
8.11.1.3 Re-define "mu" in "$\mu$m" for units (upright \mu instead of italic \mu)
8.11.1.4 Problems testing for a leap year using expl3
8.11.1.5 Vertical arrow over a whole proof
8.11.1.6 Expand \today in custom format
8.11.1.7 Is a latex3 function without a (hollow) star expandable?
8.11.1.1 Is there a LaTeX 3 transition guide for users (not gurus)?

https://tex.stackexchange.com/landing/r/digest?cta=question&id=633473

To my eternal horror a^\200\223 reading a remark by egreg to some other post a^\200\223 I discovered that LaTeX 3 has been incorporated in the standard LaTeX kernel since 2020.  I had missed it completely.  Blame the ... 
[latex3]
 
asked by ivo Welch https://tex.stackexchange.com/landing/r/digest?cta=user&id=25779 Score of 34
answered by David Carlisle https://tex.stackexchange.com/landing/r/digest?cta=user&id=1090 Score of 37
8.11.1.2 LaTeX mistake doesn't throw error

https://tex.stackexchange.com/landing/r/digest?cta=question&id=633444

I noticed today that 
\documentclass{article} 
\begin{document} 
\[ 
\Big{(a + b \Big)}^2  % yikes, note the order of the brackets! 
\] 
\end{document} 
compiles to 
How so? Shouldn't it be 
{\Big(a + b \... 
[errors]
 
asked by Nico SchlA~\[Paragraph]mer https://tex.stackexchange.com/landing/r/digest?cta=user&id=13262 Score of 11
answered by David Carlisle https://tex.stackexchange.com/landing/r/digest?cta=user&id=1090 Score of 12
8.11.1.3 Re-define "mu" in "$\mu$m" for units (upright \mu instead of italic \mu)

https://tex.stackexchange.com/landing/r/digest?cta=question&id=633647

I have a 300+ page thesis and I constantly used $\mu$, which showcases mu in italic format. Now my supervisor wants me to correct this to non-italic. I do not want to change each one-by-one, therefore ... 
[math-mode] [equations] [math-operators] [units]
 
asked by lay lay https://tex.stackexchange.com/landing/r/digest?cta=user&id=231674 Score of 9
answered by Mico https://tex.stackexchange.com/landing/r/digest?cta=user&id=5001 Score of 27
8.11.1.4 Problems testing for a leap year using expl3

https://tex.stackexchange.com/landing/r/digest?cta=question&id=633179

For a recent project I tried to become acquainted with LaTeX3 but unfortunately, I have some difficulties and cannot find the issue, which is why I'm asking for a helpful hint. 
In the project, I want ... 
[macros] [conditionals] [expl3] [latex3]
 
asked by Sam https://tex.stackexchange.com/landing/r/digest?cta=user&id=117727 Score of 9
answered by Phelype Oleinik https://tex.stackexchange.com/landing/r/digest?cta=user&id=134574 Score of 15
8.11.1.5 Vertical arrow over a whole proof

https://tex.stackexchange.com/landing/r/digest?cta=question&id=633763

Does anyone know how to draw a vertical arrow over a whole proof to demonstrate that the proof goes from the bottom to the top? 
My MWE is here: 
\documentclass{article} 
\usepackage[T1]{fontenc} 
\... 
[align] [arrows] [proofs]
 
asked by JoudaBouda https://tex.stackexchange.com/landing/r/digest?cta=user&id=230245 Score of 6
answered by egreg https://tex.stackexchange.com/landing/r/digest?cta=user&id=4427 Score of 6
8.11.1.6 Expand \today in custom format

https://tex.stackexchange.com/landing/r/digest?cta=question&id=633767

I want to fill in an already printed form. The form has boxes to write down current date. How can I expand current date so that it look like 1 4 0 2 2 0 2 2 
with a space between the digits 
[date]
 
asked by Vaibhav https://tex.stackexchange.com/landing/r/digest?cta=user&id=34083 Score of 6
answered by Steven B. Segletes https://tex.stackexchange.com/landing/r/digest?cta=user&id=25858 Score of 7
8.11.1.7 Is a latex3 function without a (hollow) star expandable?

https://tex.stackexchange.com/landing/r/digest?cta=question&id=633585

In interface3.pdf sec 1.2, it says the function with a star can be fully expandable in x, e, f, and the function with a hollow star can be fully exandable in x and e, and cannot be fully expanded ... 
[latex3]
 
asked by Syvshc https://tex.stackexchange.com/landing/r/digest?cta=user&id=180617 Score of 6
answered by Skillmon https://tex.stackexchange.com/landing/r/digest?cta=user&id=117050 Score of 7