Restrict class constructor visibility in Scala 2.9

Greetings,

How can I make the Foo constructor visible only to this package (unit test + companion object) ?

I don't want to be able to instantiate Foo outside of this 2 files...

Foo.scala

package project.foo

class Foo(val value: String)

object Foo {
  def generate: Foo = new Foo("test")
}

FooSpec.scala

package project.foo

import org.spec2.mutable._

class FooSpec extends Specification {
  "Foo" should {
    "be constructed with a string" {
      val foo = new Foo("test")
      foo.value must be "test"
    }
  }
}

I'm using Scala 2.9