--- a/scala/abacus.scala	Fri Mar 01 11:17:50 2013 +0000
+++ b/scala/abacus.scala	Fri Mar 01 23:46:02 2013 +0000
@@ -1,5 +1,6 @@
 package object abacus {
 
+import scala.annotation.tailrec
 import lib._
 
 // Abacus instructions
@@ -51,7 +52,8 @@
 
   def steps(regs: Regs, n: Int) : AConfig = steps(AConfig(0, regs), n)
 
-  def run(cf: AConfig) : AConfig = {
+  @tailrec
+  final def run(cf: AConfig) : AConfig = {
     if (cf.s >= p.length || cf.s < 0) cf else run(step(cf))
   }