onlyitems.tex

\documentclass{article}

% Based of https://tex.stackexchange.com/questions/4502/onlyitems-how-to-select-specific-items-from-an-item-list#_=_ by Ryan Reich and Yossi Farjoun

\usepackage{xparse}
\usepackage{etoolbox}

\makeatletter

\let\itemLaTeX=\item
% Have to use a \newcount because (ugh) the LaTeX \setcounter is a global assignment
\newcount\itemsSoFar
\newcommand\itemonlyitems[1][]{%
 % The \egroup closes off any \vbox'es from the previously ignored \item.
 % The conditional \bgroup cancels it out when there aren't any.
 \itemIf\bgroup\fi\egroup
 \let\itemIf=\iffalse%\fi
 \advance\itemsSoFar by 1 %
 %\itemIf
 % \def\next{\expandafter\itemLaTeX\itemOptions}%
 %\else
  % The vbox is set and then discarded, effectively ignoring an entire \item.
  % This inner \itemLaTeX is necessary to avoid a glitch when we ignore the first \item of an itemize that itself contains a nested \itemize.  Don't ask, I don't know.
 % \def\next{\setbox0=\vbox\bgroup\itemLaTeX}%
 %\fi
 \next
}

\DeclareDocumentEnvironment{ignoreitems}{ >{\ReverseBoolean} s}{
	\let\item=\itemonlyitems
	\let\itemIf=\iftrue
	\itemsSoFar = 0
	\IfBooleanTF{#1}{
		% The vbox is set and then discarded, effectively ignoring an entire \item.
		% This inner \itemLaTeX is necessary to avoid a glitch when we ignore the first \item of an itemize that itself contains a nested \itemize.  Don't ask, I don't know.
		\def\next{\setbox0=\vbox\bgroup\itemLaTeX}
	}{
		%the \@gobble s are so that various numerated lists still work as if
		%the items are missing (i.e, no side effects). reference: http://tex.stackexchange.com/questions/34811/how-to-skip-an-item-entirely-with-no-side-effects/34821#34821
		\def\next{\setbox0=\vbox\bgroup\let\stepcounter=\@gobble\let\addtocounter\@gobbletwo\itemLaTeX}
	}
}{
	\itemIf\bgroup\fi\egroup
}

\DeclareDocumentEnvironment{ignoreitems*}{}{
	\begin{ignoreitems}*
}{
	\end{ignoreitems}
}

\AtBeginEnvironment{enumerate}{\let\item=\itemLaTeX}
\makeatother

\begin{document}

\begin{enumerate}
	\item A
	\begin{ignoreitems}
		\item B
			\begin{enumerate}
				\item B1
				\item B2
			\end{enumerate}
	\end{ignoreitems}
	\item C
	\begin{ignoreitems*}
		\item D
			\begin{enumerate}
				\item D1
				\item D2
			\end{enumerate}
	\end{ignoreitems*}
	\item E
\end{enumerate}

\end{document}

\endinput