{\setbox}
{changing \box12=void}
{into \box12=
\vbox(0.0+0.0)x0.0 []}
{\unvbox}
{\setbox}
{changing \box12=void}
{into \box12=
\vbox(10.0+0.0)x0.0 []}

But the point strange to some people is this:
\setbox0\hbox{A}\begingroup\setbox0\hbox{B}\unhbox0\,endgroup\unhbox0 % shows B,A
\setbox0\hbox{A}\begingroup\unhbox0,\endgroup\unhbox0 % shows A,

/pub/mirrors/ctan/support/arara/source/src/main/java/com/github/arara

{\setbox}
{restricted horizontal mode: the letter B}
{end-group character }}
{vertical mode: \unhbox}
{horizontal mode: \unhbox}
{the character ,}
{\endgroup}
{restoring \box0=
\hbox(6.83331+0.0)x7.50002 []}
{\unhbox}

{\setbox}
{entering hbox group (level 1) at line 21}
{restricted horizontal mode: the letter A}
{end-group character }}
{leaving hbox group (level 1) entered at line 21}
{changing \box0=void}
{into \box0=
\hbox(6.83331+0.0)x7.50002 []}

\newbox\testbox
\begingroup
  \setbox\testbox=\hbox{foo}
  \begingroup
    \box\testbox
  \endgroup
  \showbox\testbox
\endgroup
\bye

!config
# TeXindy rule for arara
# requires arara 3.0+
identifier: texindy
name: TeXindy
command: <arara> texindy @{german} @{language} @{codepage} @{module} @{input} @{options} "@{getBasename(file)}.idx"
arguments:
- identifier: german
  flag: <arara> @{isTrue(parameters.german,"-g")}
- identifier: language
  flag: <arara> -L @{parameters.language}
- identifier: codepage
  flag: <arara> -C @{parameters.codepage}
- identifier: module
  flag: <arara> -M @{parameters.module}
- identifier: input
  flag: <arara> -I @{parameters.input}

% arara: texindy: { language: danish, module: 'lang/danish/utf8', input: latex }

% arara: texindy: { language: english, codepage: latin1 }

... a value of $\sim$ (4--5) $\times 10^{-11}$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$.

$\gamma_{\rm VHE}$ + $\gamma_{\rm EBL}$ $\longrightarrow$ e$^+$ e$^-$.

\noindent i)

\documentclass{article}
\usepackage{etex}
\begin{document}

a

\tracingall

\global\setbox2\hbox{abc}

{\unhbox2}

\showbox2

\stop

%%-*- texinfo -*-
%% Kannappan_hw2_prob4_function returns an N x N matrix which has ones along the
%% diagonal, -1 in the strict lower triangle and ones in the right most column
%% and zeroes everywhere else. @sp 1
%% @group
%% @sc{Input} = @var{N} (a natural number) @*
%% @sc{Output} = A N x N matrix of the pattern discussed in the docstring.@sp 1
%% @endgroup
%% @example
%% @code{Kannappan_hw2_prob4_function(3)}
%% @result{} 1   0   1
%%   -1   1   1
%%   -1  -1   1
%% @end example

Kannappan_hw2_prob4_function returns an N x N matrix which has ones
along the diagonal, -1 in the strict lower triangle and ones in the
right most column and zeroes everywhere else.

   INPUT = N (a natural number)
OUTPUT = A N x N matrix of the pattern discussed in the docstring.

     `Kannappan_hw2_prob4_function(3)'
     => 1   0   1
       -1   1   1
       -1  -1   1