+.It Fl f Ns Cm expressions-for-builtins
+Usually builtin-numbers are just immediate constants. With this flag
+expressions can be used, as long as they are compile-time constant.
+.Pp
+Example:
+.Bd -literal -offset indent
+void printA() = #1; // the usual way
+void printB() = #2-1; // with a constant expression
+.Ed