Extend applicability of \qedhere to theorem-class elements

Question

Goal: I want to be able to mark the end of examples and exercises using some sort of symbol, much as $\qedsymbol$ marks the end of proofs. In particular, I want a command that acts like \qedhere for my symbol of choice.

Background: This is in fact one of the potential modifications listed in the amsthm package documentation: to "extend applicability of \qedhere to theorem-class elements as well as to proofs." They say that while this has merit, it requires more work and is essentially on the back burner.

My attempt:

The way I have attempted to implement an exercise environment with its own end-symbol is as follows:

\documentclass{memoir}

\usepackage{amsthm}
\usepackage[varg,bigdelims]{newpxmath}
\usepackage{ifthen}

\newcounter{madesymbol}

\newtheorem{exc}{Exercise}
\newcounter{exc-counter}
\newenvironment{exercise}[1][]
{
    \begin{exc}[#1]~
    \def\mysymbol{$\lozenge$}
    \setcounter{madesymbol}{0}
    \def\tagsymbol{\stepcounter{madesymbol}\tag*{\mysymbol}}
}
{
    \ifthenelse{\equal{\value{madesymbol}}{0}}{\hspace*{\fill}\mysymbol}{}
    \end{exc}
    \stepcounter{exc-counter}
}

\begin{document}

\begin{exercise}
A function $f$...
\end{exercise}

\end{document}

This feels bulky, and I'm looking for a better solution.

Problems with ntheorem: One suggestion I have seen is the ntheorem package. I have tried ntheorem, but it seems to change "everything", e.g. not act at all like amsthm (even with the [amsthm] option loaded). If someone suggests I use this package, please give explicit code for making it act as much like amsthm as possible.

Answer 1

Try this:

\documentclass{article}
\usepackage{amsthm}
\begin{document}

\newtheorem{ex-inner}{Exercise}
\newenvironment{ex}{%
  %\def\qedsymbol{$\lozenge$}% Set the QED symbol. 
  \pushQED{\qed}%
  \begin{ex-inner}%
}{%
  \popQED
  \end{ex-inner}%
}


\begin{ex}
Here is an exercise.
\end{ex}

\begin{ex}
Hi!
\[x=y\qedhere\]
\end{ex}

\begin{proof}
Hi!
\[x=y\qedhere\]
\end{proof}
\end{document} 

Answer 2

Here is a demo of how it can be done with ntheorem. One distinctive feature is that the placement is automatic, even with therems ending in a multiline display (it may require two compilations, and there must be no blank line at the end of the environment. The QED symbol is quite easy to customise:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage[svgnames, table]{xcolor}
\usepackage{mathtools, nccmath}

\usepackage[thmmarks, thref, amsmath]{ntheorem}
\theoremheaderfont{\itshape\bfseries}% default is \upshape\bfseries
\theoremseparator{. \textemdash}
\newtheorem{thm}{Theorem}[section]
\newtheorem{prop}{Proposition}[section]

\theoremseparator{.}
\theoremheaderfont{\upshape\bfseries}%
\theorembodyfont{\upshape\mdseries}% default is \itshape
\newtheorem{dfn}{Definition}[section]
\theoremsymbol{\raisebox{-0.1\height}{\color{IndianRed}$ \boldsymbol\diamondsuit $}}
\newtheorem{ex}{Exercise}

\theoremstyle{nonumberplain}
\theoremheaderfont{\scshape}
\theoremseparator{:}
\theoremsymbol{\ensuremath{\color{Gainsboro}\blacksquare}}

\newtheorem{proof}{Proof}
\begin{document}
\setcounter{section}{2}

\begin{thm}
Clangle-Wangles’ habits of life are domestic and superfluous, and their general demeanour pensive and pellucid.
\end{thm}

\begin{dfn}
  A \textbf{Snark} is a Boojum.
\end{dfn}

\begin{ex}
Here is an exercise.
\end{ex}

\begin{ex}
Hi!
\[ x=y \]
\end{ex}

\begin{proof}
There are two cases: \useshortskip
\begin{align*}
x & =y \\ u & =v
\end{align*}
\end{proof}

\end{document}