\begin{twocollist}\itemsep=0pt
\twocolitem{\_\_WXDEBUG\_\_}{defined in debug mode, undefined in release mode}
\twocolitem{wxUSE\_XXX}{if defined as $1$, feature XXX is active
-(the symbols of this form are always defined, use #if and not #ifdef to test
+(the symbols of this form are always defined, use \#if and not \#ifdef to test
for them)}
\end{twocollist}