$ contextjit --extra=arrange --printformat=2UP --paperformat=A4*A3,landscape a.pdf

mtx-context     | processing extra: /home/ousia/ctxbeta/tex/texmf-context/tex/context/base/mtx-context-arrange.tex
mtx-context     | run 1: luajittex --fmt="/home/ousia/ctxbeta/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luajittex/cont-en" --jobname="mtx-context-arrange" --lua="/home/ousia/ctxbeta/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luajittex/cont-en.lui" --no-parse-first-line --c:currentrun=1 --c:engine="luajittex" --c:extra="arrange" --c:files="a.pdf" --c:fulljobname="/home/ousia/ctxbeta/tex/texmf-context/tex/context/base/mtx-context-arrange.tex" --c:input="/home/ousia/ctxbeta/tex/texmf-context/tex/context/base/mtx-context-arrange.tex" --c:kindofrun=1 --c:maxnofruns=8 --c:paperformat="A4*A3,landscape" --c:printformat="2UP" --c:purgeall --c:result="context-extra" "/home/ousia/ctxbeta/tex/texmf-context/tex/context/base/mtx-cont
 ext-arrange.tex"
This is LuajitTeX, Version beta-0.80.0 (TeX Live 2015/dev) (rev 5225) 
 \write18 enabled.
open source     > 1 > 1 > /home/ousia/ctxbeta/tex/texmf-context/tex/context/base/mtx-context-arrange.tex

ConTeXt  ver: 2015.04.17 02:03 MKIV beta  fmt: 2015.4.18  int: english/english

system          > 'cont-new.mkiv' loaded
open source     > 2 > 2 > /home/ousia/ctxbeta/tex/texmf-context/tex/context/base/cont-new.mkiv
close source    > 2 > 2 > /home/ousia/ctxbeta/tex/texmf-context/tex/context/base/cont-new.mkiv
system          > files > jobname 'mtx-context-arrange', input '/home/ousia/ctxbeta/tex/texmf-context/tex/context/base/mtx-context-arrange', result 'context-extra'
fonts           > latin modern fonts are not preloaded
languages       > language 'en' is active
open source     > 2 > 3 > /home/ousia/ctxbeta/tex/texmf-context/tex/context/base/mtx-context-common.tex
close source    > 2 > 3 > /home/ousia/ctxbeta/tex/texmf-context/tex/context/base/mtx-context-common.tex
close source    > 1 > 3 > /home/ousia/ctxbeta/tex/texmf-context/tex/context/base/mtx-context-arrange.tex

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(Please type a command or say `\end')