LaTeX.org

I would like an environment that saves its contents into a macro. The obvious thing
to try:

""
\newenvironment{defmacro}[0]{\def \mymacro \bgroup}{\egroup}

\begin{defmacro}
this is inside
\end{defmacro}

mymacro: \mymacro
"""

but latex says "\begin{defmacro} ended by \end{document}". Does anyone ...