\definesectionalternative[mysection][renderingsetup=mysection]

\startsetups[mysection]

  \vbox\bgroup

    \headsetupspacing

    \strut\headnumbercontent\par

    \begstrut\headtextcontent\endstrut

  \egroup

\stopsetups

\setuplabeltext[en][section={}{Section]

\setuphead[section][alternative=mysection]

It seams that my context does not know definesectionalternative

mtx-context     | run 1: luatex --fmt="/home/honza/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luatex/cont-en" --jobname="test" --lua="/home/honza/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luatex/cont-en.lui" --no-parse-first-line --c:currentrun=1 --c:fulljobname="./test.tex" --c:input="./test.tex" --c:kindofrun=1 --c:maxnofruns=8 "cont-yes.mkiv"
This is LuaTeX, Version beta-0.75.0-2013031401 (rev 4589)
 \write18 enabled.
(/home/honza/context/tex/texmf-context/tex/context/base/cont-yes.mkiv

ConTeXt  ver: 2013.03.17 13:59 MKIV  fmt: 2013.3.19  int: english/english

system          > 'cont-new.mkiv' loaded
(/home/honza/context/tex/texmf-context/tex/context/base/cont-new.mkiv
system          > beware: some patches loaded from cont-new.mkiv
)
system          > files > jobname 'test', input 'test', result 'test'
fonts           > latin modern fonts are not preloaded
languages       > language 'en' is active
(test.tex
! Undefined control sequence.
log
system          > tex > error on line 9 in file test.tex: Undefined control sequence ...


l.9 \definesectionalternative
                             [mysection][renderingsetup=mysection]
?

thanks
Jan