Tomcat 6 compilation error under Gentoo : ImportError

Installation of Tomcat 6 under Gentoo is simple : emerge tomcat.
I just met an compilation 's error :ImportError (see under).
To resolve, just follow the message in the error's description like that :
emerge -l dev-python/pyxml

>>> '/var/tmp/portage/www-servers/tomcat-6.0.20/temp/build.log'
* Messages for package www-servers/tomcat-6.0.20:
* If the output above contains:
* ImportError:
* /usr/lib/python2.4/site-packages/_xmlplus/parsers/pyexpat.so:
* undefined symbol: PyUnicodeUCS2_DecodeUTF8
* Try re-emerging dev-python/pyxml
*
* ERROR: www-servers/tomcat-6.0.20 failed.
* Call stack:
* ebuild.sh, line 49: Called src_configure
* environment, line 4310: Called java-ant-2_src_configure
* environment, line 2127: Called java-ant_bsfix
* environment, line 2149: Called java-ant_bsfix_files './build.xml' './res/deploye
/build.xml' './webapps/ROOT/build.xml' './test/build.xml'
* environment, line 2244: Called _bsfix_die 'xml-rewrite2 failed: ./test/build.xml'
* environment, line 306: Called die
* The specific snippet of code:
* die ${1} " Look at the eerror message above";
* The die message:
* xml-rewrite2 failed: ./test/build.xml Look at the eerror message above
*
* If you need support, post the topmost build error, and the call stack if relevant.
* A complete build log is located at '/var/tmp/portage/www-servers/tomcat-6.0.20/temp/build.log'.
* The ebuild environment file is located at '/var/tmp/portage/www-servers/tomcat-6.0.2
/temp/environment'.