There are various functions that are used in the built-in snippets. For example looking at the MaterialSnippets I see clamp(), tex3D_2D() and tex2Dlod(). Some of these are in the Cg documentation, like tex2Dlod. But tex3D_2D is not, and tex3D_2D seems to be doing something unusual (it's like it's overlaying 2 copies of a 2d texture on itself?)
Can you confirm that *all* of the "snippets" (not just materials?) have the Cg standard functions available?
Can you document the additional symbols/functions which we can assume are present in the snippet namespace? Alternately, is there a file somewhere that we can browse their names?
Finally, some of the input types for MaterialSnippet are not documented. The one which interests me is "Texture 3D", which I would love to use. What file format does this expect for its input? Unity does not as far as I know have a standard file format for 3D textures.