Plurality for Current Child / Tip on Parallels

[stonier]

History

Parallels came in post selectors & sequences and as a result, api had already formed around current_child and tip() for composites and subsequent tooling (visualisation) which worked on the assumption of at most one current child / tip.

Logically a parallel should have none (all failing or invalid), one or multiple current children.

Current

If there is more than one current child, then it merely points to the last of these. This behaviour (mentioned in the docstrings) pragmatically helps with visualisation / introspection (tip) and in general, hasn't been an obstruction, yet. Dealing with the ramifications of a non-singular tip is likely to be a bit of work, waiting for the justification thereof.

Future

Enable current_child and tip in all of it's plurality.