+\twocolitem{WX\_PRECOMP}{is defined if precompiled headers (PCH) are in use. In
+this case, \texttt{wx/wxprec.h} includes \texttt{wx/wx.h} which, in turn,
+includes a number of wxWidgets headers thus making it unnecessary to include
+them explicitly. However if this is not defined, you do need to include them
+and so the usual idiom which allows to support both cases is to first include
+\texttt{wx/wxprec.h} and then, inside \texttt{#ifndef WX\_PRECOMP}, individual
+headers you need.}
+\twocolitem{\_UNICODE and UNICODE}{both are defined if wxUSE\_UNICODE is set to $1$}