Nesting environment inside new command generating errors

Question

Extreme TeX novice, sorry for the length of the example, I wasn't sure what packages were important to replicate the problems. I was trying to create a simple command that I could use to injected labeled comments into an existing document. My comment would be in text color blue and be prefixed with initials ('[XXX]' in this example)

\documentclass[12pt]{article}
\usepackage{cprotect}
\usepackage{amssymb, amsmath, bm, bbm}
\usepackage{adjustbox}

\usepackage{amsmath, amsfonts}
\usepackage{bm, bbm}
\usepackage{graphics, graphicx, color, epsf}
\usepackage{natbib}
\usepackage{xr, zref, hyperref}
\usepackage{etoolbox, siunitx}
%
\usepackage{xcolor}
%
\usepackage{scrextend}
%
\newcommand\xxx[1] {\newline\textcolor{blue}{[XXX] #1}}

\begin{document}
\title{A title}
\author{A person}
\maketitle
\section{A section}
Some section stuff
\xxx{a comment}
\begin{verbatim}
This is 
text that 
presented 'as-is'
\end{verbatim}
Some more text
\xxx{
abc
\begin{verbatim}
verbatim
\end{verbatim}
}
\end{document}

Generates:

line 37: Argument of \@xverbatim has an extra }. } 
line 37: Paragraph ended before \@xverbatim was complete. } 
line 37: Extra }, or forgotten \endgroup. } 
: Emergency stop. 
: Command \@footnotemark has changed.

I tried to define a newenvironment (not shown) used by the command, thinking that perhaps the 'command' couldn't handle nested content, but it didn't help.

1. I'd like to understand what I have done wrong and how I might figure out such issues in the future.

2. Find a concise summary of the TeX syntax (not LaTeX, just basic basic TeX).

3. Understand how to get rid of that footnotemark warning. Is some package redefining that command?

Answer 1

You can probably use xparse's +v-type-argument for having LaTeX read and tokenize everything in verbatim-category-code-régime and then passing things to \scantokens for re-tokenization, but if you do this, then \xxx cannot be used within macro-arguments/macro-definitions/etc:

\documentclass[12pt]{article}
\usepackage{xparse}
%
\usepackage{xcolor}
%
\begingroup
\newcommand\xxx[1]{%
  \endgroup
  \newcommand\xxx{%
    \begingroup
    \catcode`\^^I=12 %
    \innerxxx
  }%
  \NewDocumentCommand{\innerxxx}{+v}{%
    \endgroup
    \textcolor{blue}{[XXX] %
    \begingroup
    \newlinechar=\endlinechar
    \scantokens{\endgroup##1#1}}%
  }%
}%
\catcode`\%=12\relax
\xxx{%}%

\begin{document}

\title{A title}
\author{A person}

\maketitle

\section{A section}

Some section stuff

\xxx{a comment}bla

\begin{verbatim}
This is 
text that 
presented 'as-is'
\end{verbatim}

Some more text

\xxx{%
abc
\begin{verbatim*}
verbatim
\end{verbatim*}
}%

Indenting is funny when nesting verbatim-environment in verbatim-argument:

\xxx{%
  abc
  \begin{verbatim*}
  verbatim
  \end{verbatim*}
}%

Indenting is funny when nesting verbatim-environment in verbatim-argument:

\xxx{%
  abc
  \begin{verbatim*}
  verbatim
    verbatim
\end{verbatim*}
}%

\end{document}