[Boost-build] Strange warning when building docuimentation

Subject: [Boost-build] Strange warning when building docuimentation
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-02-09 13:00:14

When building documentation within a quickbook, boostbook, doxygen 
chain, I am getting this message:

warning: Unsupported xml/html tag <_> found

from some doxygen documentation I have which reads:

"The easiest way to generate the lambda expression is to use a Boost MPL 
placeholder expression of the form 'metafunction<_>'"

In the build the message comes in this sequence (leaving out many extra 
lines):

"doxygen-action 
..\..\..\bin.v2\libs\tti\doc\msvc-10.0\debug\auto-index-internal-on\auto-index-verbose-on\auto-index-on\threading-multi\TypeTraitsIntrospectionReference-xml.xml-dir
Searching for include files...
Searching for example files...
Searching for images...
Searching for dot files...
Searching for files to exclude
Searching for files to process...
....
Generating XML output...
Generating XML output for class tti::mf_has_member_data
/tti/boost/
...
finished...
C:/Programming/VersionControl/sandbox/tti/boost/tti/TTIntrospection.hpp:1239: 
warning: Unsupported xml/html tag <_> found
xslt-xsltproc.windows 
..\..\..\bin.v2\libs\tti\doc\msvc-10.0\debug\auto-index-internal-on\auto-index-verbose-on\auto-index-on\threading-multi\TypeTraitsIntrospectionReference-xml.doxygen
xslt-xsltproc.windows 
..\..\..\bin.v2\libs\tti\doc\msvc-10.0\debug\auto-index-internal-on\auto-index-verbose-on\auto-index-on\threading-multi\TypeTraitsIntrospectionReference-xml.boostbook"

Anybody have any ideas why the sequence '<_>' in a doxygen detail block 
is producing these warnings, and which documentation building tool is 
giving the warning ? I thought it might be doxygen but that 
'finished...' line in the bjam output makes me think it is something else.