Trivial 15-line bison excercise: Implemented expressions including

user-defined numeric symbols. Whenever possible, use `expr' instead
of `NUM' to get full express ion power :-)

bird.conf

@@ -6,6 +6,8 @@
 
 router id 62.168.0.1
 
+define xyzzy = 120+10;
+
 protocol rip MyRIP_test {
-	preference 130
+	preference xyzzy;
 }