Define environment with two parts

Question

I would like to define an environment having two parts, i.e.

 \begin{myenv}
     some text
 \nextpart{myenv}
     some more
 \end{myenv}

I think I could do this by exploring the definition of \begin, \end, and \newenvironment, but I wonder if somebody has already done so.

Answer 1

I have done something similar; in my experience, it was an environment that has

1. a question
2. a proof / answer

Here's an example:

\documentclass{article}
\usepackage{amsthm}

\newenvironment{questionanswer}{
  \newcommand{\nextpart}{\end{quote}\begin{proof}}
  \begin{quote}%
  }{%
  \end{proof}%
}

\begin{document}
\begin{questionanswer}
  Something to be quoted.
  \nextpart
  Something to be proven.
\end{questionanswer}
\end{document}

To my understanding, this is expanded to exactly

\begin{document}
\begin{quote}
  Something to be quoted.
  \end{quote}\begin{proof}
  Something to be proven.
\end{proof}
\end{document}

---

Note that by using \renewcommand, this can be arbitrarily complex:

\documentclass{article}
\usepackage{amsthm}
\usepackage{amsmath}

\newenvironment{questionanswer}{%
  \newcommand{\nextpart}{\end{quote}\begin{proof}%
    \renewcommand{\nextpart}{\end{proof}\begin{itemize}%
    \renewcommand{\nextpart}{\end{itemize}\begin{enumerate}%
    \renewcommand{\nextpart}{\end{enumerate}\begin{equation}%
  }}}}% As you can see, this can be arbitrarily complex
  % (just be careful of your braces!)
  \begin{quote}%
}{%
  \end{equation}%
}

\begin{document}
Some normal text.
\begin{questionanswer}
  Something to be quoted.
  \nextpart
  Something to be proven.
  \nextpart
  \item one
  \item two
  \nextpart
  \item enum a
  \item enum b
  \nextpart
  \sum_0^\infty a_n := \lim_{n\to\infty}\sum_0^n a_n
\end{questionanswer}
\end{document}

This works by defining \nextpart to both

1. change the environment
2. change the definition of \nextpart

at each invocation. If \nextpart were defined outside of the questionsanswer environment, there'd be no way to reset \nextpart to the value it should have at the beginning of each cycle.