QED-Symbol at end of cases

Question

I have an environment >>Satz<< (German for >>theorem<<), which ends with a \diamond. But if the theorem ends with cases, the diamond is not set correctly. It is either in a new line (but at least at the right edge), or it is right behind the last character in the cases (but at least in the correct line).

Here is what it looks like:

Here is my MWE:

\documentclass{article}
\usepackage{amsmath,amsthm}
\usepackage{etoolbox}
\usepackage{cases}

\theoremstyle{definition}

\newtheorem{satz}{Satz}
\AtBeginEnvironment{satz}{%
  \pushQED{\qed}\renewcommand{\qedsymbol}{$\diamond$}%
}
\AtEndEnvironment{satz}{\popQED\endsatz}

\begin{document}

\begin{satz}
This works.
\end{satz}

\begin{satz}
This works.
\begin{align*}
f(x) &= \bigl( g(x) \bigr) \\
h(x) &= \bigl( r(x) \bigr).\qedhere
\end{align*}
\end{satz}

\begin{satz}
This
\[
\begin{cases}
    \text{does} \\ 
    \text{not}.
\end{cases}
\]
\end{satz}

\begin{satz}
And this
\[
\begin{cases}
    \text{doesn't} \\ 
    \text{either}. \qedhere
\end{cases}
\]
\end{satz}
\end{document}

Answer 1

I agree in general with what egreg says, but if you must, you can get this effect using empheq, by putting a brace to the left of an align*. (I also removed the \usepackage{cases} since pretty much anything you want to do with that package, you can also do with empheq.)

\documentclass{article}
\usepackage{amsmath,amsthm}
\usepackage{empheq}
\usepackage{etoolbox}

\theoremstyle{definition}

\newtheorem{satz}{Satz}
\AtBeginEnvironment{satz}{%
  \pushQED{\qed}\renewcommand{\qedsymbol}{$\diamond$}%
}
\AtEndEnvironment{satz}{\popQED\endsatz}

\begin{document}
\begin{satz}
This
\begin{empheq}[left=\empheqlbrace]{align*}
    & \text{does,} \\ 
    & \text{YAY}. \qedhere
\end{empheq}
\end{satz}

\end{document}