ReadonlyformSet once an event of a format it does not know arrived: it follows that source no more.
Let a tree go (a finished log a page no longer shows): it keeps nothing of it.
Whether it may resume in place from source (streaming.md, "Resuming"):
when the source can give every event it holds as it holds it. Always
for the kept form; for a live form, only from the process that gave it
every event it holds.
Take one event, as received. from: which process gave it (a writer's
number), or "store"; a live reader's default is the event's writer.
Read a tree again from a source (after a loss, a reconnection): after its
last event when it may resume in place, and from the beginning when it
may not or the source answers event-unknown (dropping what it held).
Returns the reads it made, in order. A source it can read lets a stopped
reader follow again; an event of a format it does not know stops it
there (it takes nothing from that event on).
A reader following one form of logs live (streaming.md, "Following a log"), keeping for each tree the events it holds.
form:"kept"(the kept form, or a view made from it) or"live"(the whole log, or a view of it that may show values the kept form lacks); it decides where the reader may resume in place.