A walk over a large library every hour loads the array that protects it. An item now says how it
is measured — a bounded daily walk, a dataset's counters, or its top level only — and a size that
is a lower bound is kept as such and never read as a shrink.
A module's data section says what it keeps and how precious it is; the backup holder's lines,
binding stickiness, retirement on unassign and D13's conditions follow from it, so issue 273's
empty replacement is said and an unassigned module's data is remembered, not forgotten.
jochen