[OS X TeX] \setminus in Lucida

[Ulrich Groh]

The problem: 

\documentclass{scrartcl} 
%% 
% 
\usepackage{fontspec}      % support opentype text fonts 
\usepackage{unicode-math}  % support opentype math fonts 
\unimathsetup{partial=upright} 
%% 
\setmathfont{LucidaBrightMathOT.otf} 
\setmathfont[range=\mathfrak]{Latin Modern Math} 
\setmathfont[range=\mathcal]{Latin Modern Math} 
% 
% 
\renewcommand{\setminus}{\smallsetminus} % nothing happens 
\renewcommand{\imath}{\, \mathrm{i} \, } 
%% 
% 
\begin{document} 
% 
\renewcommand{\setminus}{\smallsetminus} % ok 
\renewcommand{\imath}{\, \mathrm{i} \, } 
% 
\[ 
     A \setminus B  \, , \quad   x + \imath y = z \, . 
\] 
% 
\end{document} 

If I compile the TeX-file with LuaLaTeX there is the  (small)setminus-symbol an slso i (=mathrm-i) 

But if I compile before the  \begin{document} nothing happens, i.e. no \setminus-symbol and no \imath-symbol. Why?

Ulrich