btx/xml tree - entry with missing field

[Michael Eidenbenz <eidenbenz@arch.ethz.ch>]

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Hello

I use the xml interface to list all publications that dont match a search criteria:

     /field[@name='keywords'
            and not (contains(text(),'alpha'))]

but there are entries where the keywords field is missing.
how can I include them (here "test-3") in the output? 

Thanks Michael

--------------------------

\startbuffer[bib]

@article{test-1,
   title    = {Article One},
   author   = {A. Foo and X. Bar},
   year     = {2001},
   keywords = {alpha},
}

@book{test-2,
   title    = {Book Two},
   author   = {B. Foo},
   year     = {2002},
   keywords = {beta},
}

@whatever{test-3,
   title    = {Book Three},
   author   = {C. Foo},
   year     = {2003},
}

\stopbuffer

\usebtxdataset [database] [bib.buffer]
\convertbtxdatasettoxml [database]

\startxmlsetups btx:tag
\hbox{\xmlatt{#1}{tag}}
\stopxmlsetups

\startxmlsetups btx:demo
   \xmlfilter {#1} {
     /bibtex
     /*
     /field[@name='keywords'
            and not (contains(text(),'alpha'))]
     /..
     /command(btx:tag)
   }
\stopxmlsetups

\starttext
\xmlsetup{btx:database}{btx:demo}
\stoptext

Michael Eidenbenz / Senior Researcher
eidenbenz@arch.ethz.ch <mailto:eidenbenz@arch.ethz.ch>
Phone +41 44 632 33 46

Chair of Architecture and Building Process
ETH Zurich / Building HIB / Floor E / Room 33
Stefano-Franscini-Platz 1 / CH-8093 Zurich
www.bauprozess.arch.ethz.ch <http://www.bauprozess.arch.ethz.ch/>

/ ITA
Institute of Technology in Architecture
Faculty of Architecture / ETH Zurich
www.ita.arch.ethz.ch <http://www.ita.arch.ethz.ch/>


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