[luatex] a problem with luatexUmathcode

[jfbu]

Hi,  [I just posted on c.t.tex, but figured here could be better]

the following code gives dissimilar results for XeTeX 
and LuaLatex. 

\documentclass{article} 
\usepackage{ifxetex} 
\begin{document} 
\ifxetex 

\the\mathcode`\-           % 8704="2200 
\the\XeTeXmathcodenum`\-   % 37748736=2.2^{24} + 2.2^{21} + 0 

\else 

\the\mathcode`\-           % 8704 = "2200 

\the\luatexUmathcodenum`\- % 8704, if interpreted as Unicode 
                           % mathcode 0.2^{24} + 0.2^{21} + 8704 
\fi 
\end{document} 

This is a bit strange. The problem with this behavior of LuaLaTeX 
is that I need to know *in advance* if I have to interpret the 
number resulting from \luatexUmathcodenum as a legacy mathcode 
or as a Unicode mathcode ! ... 

best wishes, 
Jean-Francois