How does one go about using the ashikhmin bsdf? I see it listed here. http://www.sidefx.com/docs/houdini11.0/vex/pbr [sidefx.com]

I expected the option to be present in the lighting model node but that's a no go. Is it hidden? is it a node at all? do I have to write it? I'm confused. Any help would be appreciated.