Replace an arc with other arc(s).

[mbostock]

It should be possible to take one arc (the source arc) and replace it with the corresponding subset of another arc (the target arc). For example, here replacing the light blue arc with the similar subset from the darker blue arc:

This feature is already implemented, but it lacks some functionality:

• In some cases, the target arc lacks control points near the endpoints of the source arc. It will be necessary to introduce new control points in the target arc so that the appropriate subset can be cut out of the target arc. (This will also require detecting coincident arcs.)
• Sometimes there is not a single target arc that is a superset of the source arc, but multiple target arcs that we want to use to replace the source arc. One solution to this may be to split the source arc first, so that we replace a subset of the source arc with the corresponding subset of the target arc, and then the next subset of the source arc with the corresponding subset of the next target arc, and so on. Alternatively, a way of replacing a source arc with a set of target arcs, and splitting the source arc accordingly.
• Similarly, sometimes we just want to replace a subset of a source arc with a subset of a target arc, thus requiring us to split the source arc before replacement.