\documentclass{article}
\begin{document}
\ExplSyntaxOn
\keys_define:nn {my}{
 key   .code:n = \exp_args:Nnx \keys_set:nn{my}{__key=#1},
 __key .choice: ,
 __key / choicea .code:n = A
}
\newcommand\testA {choicea}
\keys_set:nn {my} { key = choicea}
\keys_set:nn {my} { key = \testA}
\keys_set:nn {my} { key = blub}
\ExplSyntaxOff

\end{document}

[ 2/15, 00:03/03:59] update: arara [15567k] (54089 -> 54121) ... done

$ arara --version
/usr/local/texlive/2020/bin/x86_64-linux/arara: 17: exec: java: not found

Command 'java' not found, but can be installed with:

sudo apt install default-jre
sudo apt install openjdk-11-jre-headless
sudo apt install openjdk-8-jre-headless

davidc@dc-bantham:/mnt/c/tmp$ arara --version
  __ _ _ __ __ _ _ __ __ _
 / _` | '__/ _` | '__/ _` |
| (_| | | | (_| | | | (_| |
 \__,_|_|  \__,_|_|  \__,_|

arara 5.0.1
Copyright (c) 2020, Island of TeX
arara is released under the New BSD license.

New features in version 5.0.1:
* Introduced working directory support.
* Reorganized code base for more speed.
* Removed triggers and introduced session namespaces.
* Removed the velocity templating language and several outdated methods.
* Removed string-based commands (return "command";).

C:\Users\Nililand2020\Documents\Tests>arara --version
Exception in thread "main" java.lang.ExceptionInInitializerError
        at sun.misc.Unsafe.ensureClassInitialized(Native Method)
        at sun.reflect.UnsafeFieldAccessorFactory.newFieldAccessor(Unknown Source)
        at sun.reflect.ReflectionFactory.newFieldAccessor(Unknown Source)

C:\Users\Nililand2020\Documents\Tests>java -version
java version "1.8.0_241"
Java(TM) SE Runtime Environment (build 1.8.0_241-b07)
Java HotSpot(TM) Client VM (build 25.241-b07, mixed mode)

Exception in thread "main" java.lang.ExceptionInInitializerError
        at sun.misc.Unsafe.ensureClassInitialized(Native Method)
        at sun.reflect.UnsafeFieldAccessorFactory.newFieldAccessor(Unknown Source)
        at sun.reflect.ReflectionFactory.newFieldAccessor(Unknown Source)
        at java.lang.reflect.Field.acquireFieldAccessor(Unknown Source)
        at java.lang.reflect.Field.getFieldAccessor(Unknown Source)
        at java.lang.reflect.Field.get(Unknown Source)
        at kotlin.reflect.jvm.internal.KClassImpl$Data$objectInstance$2.invoke(KClassImpl.kt:114)

\documentclass{scrartcl}

\makeatletter
\scr@ifpdftex{\typeout{yes}}{\typeout{no}}
\begin{document}

\end{document}

Package scrbase Warning: Usage of deprecated command `\ifnotundefined'.
(scrbase)                The command has been renamed because of a
(scrbase)                recommendation of The LaTeX Project Team.
(scrbase)                Please replace `\ifnotundefined' by `\Ifnotundefined'

\documentclass[]{article}

\usepackage[]{amsmath}

\usepackage[]{hyperref}

\begin{document}
\begin{subequations}\label{eq:a}
\begin{align}
  a &= b \\
  c &= d \\
  e &= f
  \intertext{Text.}
  \refstepcounter{parentequation}\label{eq:A}
  A &= b \\
  C &= d \\
  E &= f
\end{align}
\end{subequations}
\end{document}