How to automatically add text immediately after \begin{document}

Question

I attempted to redefine the document environment as follows, but get a complaint that:

LaTeX Error: \begin{MYdocument} on input line 13 ended by \end{document}.

\documentclass[12pt]{article}
\newtoks{\SpecialText} 
\SpecialText={Text to be printed after begin document}

\newenvironment{MYdocument}{%
  \begin{document}{}
  Note: \the\SpecialText
}{%
  \end{document}
} 

\begin{MYdocument}
 ... file contents ...
\end{MYdocument}

I want this to produce the following output

Note: Text to be printed after begin document

... file contents ...

If I could do this by just slightly modifying the environment that would be preferable so that I could just use \begin{document} ... \end{document}.

Answer 1

You can use the \AtBeginDocument command before the \begin{document} line to queue up code (even multiple pieces of code) so that it it executed when that line is hit. Here's a reference.

Answer 2

As already stated by Aaron, \AtBeginDocument{<code>} can be used to place code "at the begin of the document". This hook is executed after the old .aux is read in and the new one is opened, but before the preamble is officially closed. Therefore the code can still load packages and add already type-settable material which writes into the .aux file, like sectioning commands. So it is actually executed with one foot in the preamble and the other one in the document body.

The etoolbox package provides \AfterEndPreamble{<code>} (and other useful hooks) which is executed after the preamble is closed, so e.g. all preamble-only macros are disabled etc. Actually it is executed at the very end of \begin{document}, only followed by an \ignorespaces. I personally would prefer \AfterEndPreamble to \AtBeginDocument (already heavily used by packages) to add code at the very beginning of the document body.