--- a/scala/turing.scala	Sat Mar 02 10:42:39 2013 +0000
+++ b/scala/turing.scala	Sun Mar 03 00:18:13 2013 +0000
@@ -65,7 +65,7 @@
 
   // composition
   def ++ (that: TM) = TM(this.p ::: that.p)
-  def :+ (that: TM) = this.adjust ++ that.shift(this.p.length / 2 + 1)
+  def :+ (that: TM) = this.adjust ++ that.shift(this.p.length / 2)
 
   def shift(n: Int) =
     TM(p.map{case (a, s) => (a, if (s == 0) 0 else s + n)})