--- a/progs/app51.scala	Tue Mar 22 17:09:24 2016 +0000
+++ b/progs/app51.scala	Wed Apr 06 11:51:33 2016 +0100
@@ -1,7 +1,7 @@
-def OPT(r: Rexp) = ALT(r, EMPTY)
+def OPT(r: Rexp) = ALT(r, ONE)
 
 def NTIMES(r: Rexp, n: Int) : Rexp = n match {
-  case 0 => EMPTY
+  case 0 => ONE
   case 1 => r
   case n => SEQ(r, NTIMES(r, n - 1))
 }