The following shader doesn't work as expected in Arnold 5. There should be both a strong forward and strong backscatter effect, but only the last shader's phase value (g) is used:
volume sample_volume_shader ( output closure color result = 0 )
{
closure color a = volume_henyey_greenstein(0, 1, 0, -0.8);
closure color a = volume_henyey_greenstein(0, 1, 0, 0.8);
result = a + b;
}