Continuing an earlier theorem-like environment with amsthm & thmtools

Question

When using amsthm, I want to be able to continue a particular theorem-like environment (in my case, an example) later. The continued environment should have the same number as the one it continues from earlier; and after the continued environment is finished, normal theorem numbering should resume.

The following accomplishes what I want. Is there a simpler or otherwise better way -- without having to stray below LaTeX into pure TeX?

% continue-thm.try
\documentclass[12pt]{article}

\usepackage{amsthm}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{proposition}[theorem]{Proposition}
\theoremstyle{definition}
\newtheorem{example}[theorem]{Example}

\newcounter{savedthmcounter}
\newcounter{tobecontinuedthmcounter}

% Invoke \tobecontinued immediately AFTER theorem to be continued
\newcommand{\tobecontinued}{%
    \setcounter{tobecontinuedthmcounter}{\value{theorem}}%
    \addtocounter{tobecontinuedthmcounter}{-1}}
% Invoke \continue immediately BEFORE continuing theorem that was to be continued
\newcommand{\continue}{%
    \setcounter{savedthmcounter}{\value{theorem}}%
    \setcounter{theorem}{\value{tobecontinuedthmcounter}}}
% Invoke immediately AFTER done with the continued theorem
\newcommand{\resume}{\setcounter{theorem}{\value{savedthmcounter}}}

\begin{document}

\begin{theorem}% 1
Theorem 1.
\end{theorem}

\begin{example}% 2
\label{ex:ex2}
An example.
\end{example}
%
\tobecontinued

\begin{theorem}% 3
Theorem 3.
\end{theorem}

\begin{theorem}%4
Theorem 4.
\end{theorem}

\begin{proposition}
A proposition.
\end{proposition}

\continue
%
\begin{example}[continued]% 2 again
Example \ref{ex:ex2}, continued.
\end{example}

\resume

\begin{theorem}% 5
Theorem 5.
\end{theorem}

\end{document}

Added

The preceding method works not just for amsthm with article, etc., but also when I use thmtools and then \declaretheoremstyle and \declaretheorem(and, as it happens, with the memoir class that I'm using).

How do I invoke the thmtools package's \declaretheoremstyle and \declaretheorem (with memoir) so as to adapt the proposed answers -- and most especially egreg's answer?

Update: If the theorem-like environment to be continued already has a note (i.e., from something like \begin{example}[some text]), then a method different from the above and from the accepted answer, below, seems to be required. One such method is shown in https://tex.stackexchange.com/a/702575/13492.

Answer 1

I don't think you need such a complex framework: I can only think to continuation of examples.

\documentclass[12pt]{article}

\usepackage{amsthm}

\newtheorem{theorem}{Theorem}[section]
\newtheorem{proposition}[theorem]{Proposition}

\theoremstyle{definition}
\newtheorem{example}[theorem]{Example}
\newtheorem*{excont}{Example \continuation}
\newcommand{\continuation}{??}
\newenvironment{continueexample}[1]
 {\renewcommand{\continuation}{\ref{#1}}\excont[continued]}
 {\endexcont}

\begin{document}

\begin{theorem}% 1
Theorem 1.
\end{theorem}

\begin{example}% 2
\label{ex:ex2}
An example.
\end{example}

\begin{theorem}% 3
Theorem 3.
\end{theorem}

\begin{theorem}%4
Theorem 4.
\end{theorem}

\begin{proposition}
A proposition.
\end{proposition}

\begin{continueexample}{ex:ex2}
Example \ref{ex:ex2}, continued.
\end{continueexample}

\begin{theorem}% 5
Theorem 5.
\end{theorem}

\end{document}

Just give a \label to the example you want to continue and use it as the argument to continueexample.

With thmtools it's no different, just remember to use \protect\continuation.

\documentclass[12pt]{article}

\usepackage{amsthm}
\usepackage{thmtools}

\declaretheoremstyle[
  bodyfont=\upshape,
]{upshape}

\declaretheorem[
  name=Theorem,
  within=section,
]{theorem}
\declaretheorem[
  name=Proposition,
  sibling=theorem,
]{proposition}
\declaretheorem[
  style=upshape,
  name=Example,
  sibling=theorem,
]{example}
\declaretheorem[
  style=upshape,
  name=Example \protect\continuation,
  unnumbered,
]{excont}
\newcommand{\continuation}{??}
\newenvironment{continueexample}[1]
 {\renewcommand{\continuation}{\ref{#1}}\excont[continued]}
 {\endexcont}


\begin{document}

\begin{theorem}% 1
Theorem 1.
\end{theorem}

\begin{example}% 2
\label{ex:ex2}
An example.
\end{example}

\begin{theorem}% 3
Theorem 3.
\end{theorem}

\begin{theorem}%4
Theorem 4.
\end{theorem}

\begin{proposition}
A proposition.
\end{proposition}

\begin{continueexample}{ex:ex2}
Example \ref{ex:ex2}, continued.
\end{continueexample}

\begin{theorem}% 5
Theorem 5.
\end{theorem}

\end{document}

Answer 2

Based on https://tex.stackexchange.com/a/311985/36296 you could create a new environment which automatically get the previous example number from the label and resets it after your continued example. This will

• save you from having to manually add \tobecontinued, \continue and \resume

• works for every example, not just the last one marked with \tobecontinued

---

% continue-thm.try
\documentclass[12pt]{article}

\usepackage{amsthm}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{proposition}[theorem]{Proposition}
\theoremstyle{definition}
\newtheorem{example}[theorem]{Example}

\newcounter{savedthmcounter}
\newcounter{tobecontinuedthmcounter}

\makeatletter
\def\@extractthmnum#1.#2{#2}
\newcommand{\extractthmnum}[2]{% \extractthmnum{<cnt>}{<ref>}
  \setcounter{#1}{0}% Default
  \ifcsname r@#2\endcsname
    \edef\@tempa{\csname r@#2\endcsname}% Extract complete reference
    \edef\@tempa{\expandafter\@firstoftwo\@tempa}% Extract number
    \setcounter{#1}{\expandafter\@extractthmnum\@tempa}% Strip number
  \fi
}
\makeatother

\newenvironment{excont}[1][]{
    \extractthmnum{tobecontinuedthmcounter}{#1}
    \addtocounter{tobecontinuedthmcounter}{-1}
     \setcounter{savedthmcounter}{\value{theorem}}
    \setcounter{theorem}{\value{tobecontinuedthmcounter}}
    \begin{example}[continued]
}{
\end{example}
\setcounter{theorem}{\value{savedthmcounter}}
}

\begin{document}

\begin{theorem}% 1
Theorem 1.
\end{theorem}

\begin{example}% 2
\label{ex:ex2}
An example.
\end{example}


\begin{theorem}% 3
Theorem 3.
\end{theorem}

\begin{theorem}%4
Theorem 4.
\end{theorem}

\begin{proposition}
A proposition.
\end{proposition}

\begin{excont}[ex:ex2]% 2 again
Example \ref{ex:ex2}, continued.
\end{excont}


\begin{theorem}% 5
Theorem 5.
\end{theorem}

\end{document}