Merge adr009/zset (d1665f64) into adr009/derived

context/code/zset.md

@@ -59,9 +59,8 @@ These are the readings this step takes.
    is required only where a result copies keys (`filter`, `distinct`, `scale`, the borrowed
    operators); `map` and the by-value operators need only `Ord`.
 5. **Overflow.** Neither clause 1 nor clause 8 states a policy for weights (clause 8 is about
-   `f64` sums). A weight that wraps would turn a retraction into an insertion with nothing
-   to show it, which is the failure the ADR's exactness argument is against. So every weight
-   operation uses `checked_*` and **panics** with `zset: weight overflow in <op>`, in debug
+   `f64` drift, which wrapping integers do not have). The crate currently uses `checked_*`
+   on every weight operation, for the reason given below, and **panics** with `zset: weight overflow in <op>`, in debug
    and in release (`checked_*` does not depend on `overflow-checks`). The operations are
    `add` (including `insert`, `Extend`, `FromIterator`), `sub`, `neg` (so `i64::MIN` panics
    when it has to be negated) and `scale`. Scaling by zero cannot overflow and returns the
@@ -79,7 +78,8 @@ These are the readings this step takes.
    under wrapping. (An earlier version of this paragraph said otherwise; that was wrong.) What
    wrapping breaks is the intermediate reads. `MAX + 1` stores `i64::MIN`, so `distinct`
    drops a key whose true weight is positive and `weight()` has the wrong sign. A true weight
-   of 2^64 stores as 0, so the key vanishes and `is_zero`, `len` and `==` report a set that
+   of 2^64 stores as 0 (by the add path; a `scale` product of 2^64 would store a zero too, which
+   breaks canonical form), so the key vanishes and `is_zero`, `len` and `==` report a set that
    does not exist. Those are wrong answers returned silently, which is why the crate panics
    instead. A wider weight (`i128`) moves the boundary and does not remove it. The ADR's
    "abelian group" wording needs amending whichever policy is chosen: checked-and-panic gives