public void PermanentVariables()
{
EnvironmentFrame f = new EnvironmentFrame(null, null, 5);
Assert.AreSame(f["Y0"], f.PermanentVariables);
Assert.AreSame(f["Y1"], f.PermanentVariables.Next);
Assert.AreSame(f["Y2"], f.PermanentVariables.Next.Next);
Assert.AreSame(f["Y3"], f.PermanentVariables.Next.Next.Next);
Assert.AreSame(f["Y4"], f.PermanentVariables.Next.Next.Next.Next);
Assert.IsNull( f.PermanentVariables.Next.Next.Next.Next.Next);
}