Pandoc - Support for a custom input latex environment - latex

Question

I am using a single unified LaTeX doc to create problem sets and solutions:
\item What is one plus one?
\begin{soln}
The answer is "two".
\end{soln}
In LaTeX, I define this environment with (simplified):
\NewEnviron{soln}
{
\ifsolutions\expandafter
\BODY
\fi
}
That is, if \solutionsfalse has been defined in LaTeX, it prints:
1. What is one plus one?
and if \solutionstrue has been defined, it prints:
1. What is one plus one?
** The answer is two **
I'm trying to replicate this in pandoc to generate HTML or MD files from the latex input, but I've run against the wall. Pandoc doesn't honor any kind of /if /else /fi statement in LaTeX, I think. Pandoc doesn't honor the comment environment, which would also work with \excludecomment{soln}. So, I can't come up with a shim.tex file that would replicate the 'ignore stuff in the soln environment'.
The next way to go would, I guess, be to do something in luatex that pandoc can talk to, or to define the custom environment to pandoc with a filter? But the documentation for those systems is extremely heavyweight - there's no easy way in.
Can anyone suggest a solution to this?
Ideally, I want to run two different shell commands. Command A should omit all content in the soln environment. Command B, ideally, should turn all regular text blue, and show all content in the soln environment in black color.
(P.S. The xcolor package also seems unsupported in native pandoc, although there is a filter that doesn't work for me.)
Edit
Following comments by #tarleb and #mb21, I guess I have to try to work out how filters work. Again, the documentation here is terrible - it wants you to know everything before you can do anything.
I tried this:
return {
{
RawBlock = function(elem)
print(elem.text)
if starts_with('\\begin{soln}', elem.text) then
return pandoc.RawBlock(elem.format,"SOLN")
else
return elem
end
end,
}
}
and ran it with
pandoc --lua-filter ifdef.lua --mathjax -s hw01.tex -s -o hw01.html
But there is nothing on stdout from the print statement, and my document is unchanged, so the RawBlocks are apparently not processed by the lua filter unless the -f latex+raw_tex flag is passed. But passing those flags means that pandoc doesn't actually process the \include commands in the latex, so my filter wont' see the subdocuments.
Apparently, the answer is "No, pandoc cannot support new latex environment", because it would require modifying the parser. Although the -f latex+raw_tex can disable big parts of the parser, that just means the document is largely unparsed, which isn't what I want.
Please tell me if I'm wrong.