diff -r ed1b6b8c36e5 -r 0b727d1a8184 main_testing4/knight_test4.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/main_testing4/knight_test4.scala	Mon Nov 02 02:31:44 2020 +0000
@@ -0,0 +1,6 @@
+import CW9a._
+
+val f_urban = (x:(Int, Int)) => if (x._1 > 3) Some(List(x)) else None
+
+assert(first(List((1,0),(2,0),(3,0),(4,0)), f_urban) == Some(List((4,0))))
+assert(first(List((1,0),(2,0),(3,0)), f_urban) == None)