Some private defns make extending `emit` harder than necessary

[bolivier]

The following methods are private and, in my view, offer useful behavior for extending emit, which you'd need/want to do if you used a custom parser.

• empty-or-div
• emit-style
• tag->tag+id+classes
• emit-attrs

I'm specifically interested in emit-attrs and emit-style, but I think the other two could easily be useful in some situations.