+5. a {\bf indeterminate-if} b
+
+For example, wxUSE_UNICODE indeterminate-if Custom.
+
+\begin{verbatim}
+ a b
+
+ ? 1
+\end{verbatim}
+
+This overrides all the other dependencies, and allows you
+to make an option user-choosable in some circumstances,
+when otherwise it would be constrained to be either enabled or
+disabled. You may need to use an intermediate option to
+make sensible use of this: for example make the intermediate
+option dependent on a number of factors, such as Unicode not being
+available on some platforms.
+