The environment inside the new environment

Question

There was a need to create an environment that uses a different environment inside. I can implement this, for example, like this:

\newenvironment{new}{ \begin{old} }{ \end{old} }

And everything works fine until old becomes document. In this case, the code stops working:

! LaTeX Error: \begin{Document} on input line 2 ended by \end{document}

Why does the above code work fine in the case of non-document, but not in the case of document?

P.S. I will also give a solution that was found on the vastness of this site:

\newenvironment{Document}{}{}
\AddToHook{env/Document/before}{ \begin{document} }
\AddToHook{env/Document/after}{ \end{document} }

Answer 1

There is a very good reason for this approach not be working.

Every environment makes a group, with one exception, namely document. Why? Because typesetting the whole document inside a group would quite easily lead to filling the memory, for TeX would need to retain every assignment in order to restore it at the end of the group (and for no purpose, since the document has ended).

Besides the approach not being workable, I can't see any good reason for hiding document inside another environment, because this would lead to exactly the same problem the LaTeX kernel wants to avoid.