Summary

Key classes and roles:

List         Iterable
Array        Iterator
Parcel       ListIter
Positional   MapIter
Range        GatherIter
Nil

Contexts:

list context - interpolates Iterables
flat context - also interpolates Parcels
item context - prevents flattening
lol  context - creates lists-of-lists

Iterator immutability key to avoiding action-at-distance

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