I'm glad you asked!

While @eford1 was on vacation, I changed the names of everything. The updated names are in the shepherd readme. 
- "optional" was changed to "void", because the primary purpose of these nodes was to keep them from showing up as arguments to the callback
- "silent" was changed to "important", because the primary purpose of these nodes was to execute them before other nodes (like CSS !important)

i don't have my heart set on these particular names, but i do think they're much better than the old names