[tex4ht] Blackboard bold

[Hans Georg Schaathun]

Dear all,

Does anyone know of a good way to get blackboard bold in MathML
output from TeX4ml?  All I need are the N/Z/Q/R/C for natural
numbers, integers, and rational, real and complex numbers.

I found a list of suitable MathML tags at
   http://www.w3.org/TR/MathML2/double-struck.html
so I am certain it can be done.  Since I am worried that I will
mess up the MathML if I try to jury rig my macros, it would be nice 
if someone has found an out-of-the-box solution.

TIA
-- 
:-- Hans Georg