Note: many of the slides showing simple shaders didn't include the parameter type declaration, e.g.:
output color_Pref = color(0.0)
instead of
output color color_Pref = color(0.0)
Unfortunately, I copy-pasted these simple code examples into many of the slides. This error has been corrected in the upcoming Siggraph 2023 course presentation: Shader Writing in Open Shading Language.