Counting array appends as edge-preserving makes `keep(item, items) { items.push(item); return item; }` a borrowed-return function: it preserves every heap edge, borrows every reference parameter and returns one of them. Its borrowing body now retains only the appended element and returns the parameter +0, and the owned adapter retains the result before releasing its parameters. An append never releases an element (the slow path stores past the old length), so the caller's owner keeps the result alive.
Malte Ubl
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