#353 I have the breadcrumb undo and redo working. Mapping arcs after a mapping expansion are not being re-added as normal arcs, but instead only as temporary hover-only arcs. This is the usual behavior for mapping arcs that were not the direct result of a mapping expansion. I will either have to trigger some behaviors specialized to mapping expansions when I redo a mapping expansion set, or I will need to adjust some logic in arc manifestation.