Q: Is there an easy way to change the default `proof`

environment endmark (it's normally a funny open square and I'd like to change it to `\blacksquare`

) when using the `ntheorem`

package with the `amsthm`

and `thmmarks`

options? I'd like to continue using the `ntheorem`

package without migrating over to `amsthm`

You can see below that I added a `\square`

endmark to the different theorem environments. I'm just stuck on changing the `proof`

endmark. You can see below I tried the code

`\renewcommand{\qedsymbol}{\ensuremath{\blacksquare}}`

It didn't work. A number of other approaches didn't work either.

```
\documentclass{article}
\usepackage[amsthm,thmmarks]{ntheorem}
\usepackage{amssymb}
\theoremstyle{break}
\theoremsymbol{\ensuremath{\square}}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{definition}[theorem]{Definition}
%\renewcommand{\qedsymbol}{\ensuremath{\blacksquare}}
\begin{document}
\begin{theorem}[My Theorem]
Lorem ipsum.
\end{theorem}
\begin{proof}
Lorem ipsum.
\end{proof}
\begin{definition}[My Definition]
Lorem ipsum.
\end{definition}
\end{document}
```

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