-When you're looking at the past, edits to the current notebook don't change what you're seeing.
-Factored out in-present? and relevant-event? to their own predicates (had a bunch of repetition happening earlier, and this made it a bit more obvious)
-First crack at surgical forking
-Added event-listener on history-slider in the HTML rather than after the fact
-Factored out some of the common stuff between notebook rendering in notebook! and the surgical modifications needed in post/fork
-notebook! now calls the new function surgical! to avoid overlap