A question's evidence epoch and declaration describe that question and end with
it, so `begin_question` clears them. Held past it, they let the last question's
citations ground the next one and hold a horizon its own declarations cannot
pass, and they force every question's message count to be comparable to counts
taken over a history that a host may since have rebuilt: a thread reconstructed
without a run that never finished shifts every later position, and the next
question was refused where answering it is correct.
Identities still separate one question from the next. Occurrences outlive the
question that recorded them and carry the identities that cited them, which the
capsule is grouped and ordered by, so reuse merges two questions into one group
and a lower identity renders later evidence as earlier.
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