"Is Final Render," output on "light path," or "camera data," node.

Something I was thinking about today which would be handy when designing shaders in cycles would be a "Is Final Render," output on the light path node. This would output true if rendering through F12, and false if rendering using the viewport. This could be used for when you are previewing textures or certain parts of the shader graph, but want the output render to be the complete shader.