Strange/unexpected unsafePerformIO behavior

blue_in_the_face.scala

import scalaz._
import Scalaz._
import scalaz.effects._
val a = println("a here").pure[IO]
val b = println("b here").pure[IO]
val c = a.flatMap(_ => b.flatMap(_ => println("c here").pure[IO]))
c.unsafePerformIO // prints 3 lines as expected
c.unsafePerformIO // expect 0 lines to be printed, but...
c.unsafePerformIO // and again...
c.unsafePerformIO // again...
c.unsafePerformIO // ad infinitum

repl.scala

Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_26).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import scalaz._
import scalaz._

scala> import Scalaz._
import Scalaz._

scala> import scalaz.effects._
import scalaz.effects._

scala> val a = println("a here").pure[IO]
a: scalaz.effects.IO[Unit] = scalaz.effects.IO$$anon$2@26c46c5a

scala> val b = println("b here").pure[IO]
b: scalaz.effects.IO[Unit] = scalaz.effects.IO$$anon$2@1f46e809

scala> val c = a.flatMap(_ => b.flatMap(_ => println("c here").pure[IO]))
c: scalaz.effects.IO[Unit] = scalaz.effects.IO$$anon$2@46249be9

scala> c.unsafePerformIO // prints 3 lines as expected
a here
b here
c here

scala> c.unsafePerformIO // expect 0 lines to be printed, but...
c here

scala> c.unsafePerformIO // and again...
c here

scala> c.unsafePerformIO // again...
c here

scala> c.unsafePerformIO // ad infinitum
c here

scala>