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}