I tried putting my keyboard where my mouth was and failed...here's what I manged:
\edef\myanswers{\relax}
\def\mysow#1{\edef\myanswers{\myanswers\ #1} }
\def\myreap{\myanswers}
It works, but not when I put \begin{...}...\end{...}
environments into it...I've set the answer CW...perhaps someone knows how to make it work so that you can put environments into that...or perhaps it isn't as simple as I thought.
Here's something that passes the \begin{...} ... \end{...}
test:
\documentclass{article}
\makeatletter
\newcommand{\mysow}[1]{%
\@ifundefined{myreap}{%
\def\myreap{#1}%
}{%
\g@addto@macro{\myreap}{,\ #1}%
}}
\makeatother
\begin{document}
You \mysow{reap} what \mysow{you} sow.
\mysow{%
\begin{enumerate}
\item You reap what you sow.
\end{enumerate}
}
\myreap
\end{document}
But there may well be more cases where it fails.