@@ -103,9 +103,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `NotSupportedException` on a sorted set and have no function.
 * `add`, `addRange`, `remove`, `removeRange`, `clear`, `update`, `filter` and `partition` of `SortedSet` keep the comparer
   of their input, and the set algebra keeps the comparer of the first sorted set. Every function throws
-  `ArgumentNullException` for a `null` sorted set, sequence or array. `unionMany` of an empty sequence returns an empty
-  sorted set, as `Set.unionMany` does. `tryGetValue` returns a `voption`, `choose` takes a function that returns a
-  `voption`, and `partition` and `partitionWith` return struct tuples.
+  `ArgumentNullException` for a `null` sorted set, sequence or array, and for a default flat list that is given as a
+  sequence; `yield!` and `for` of `sortedSet { }` do the same. `unionMany` of an empty sequence returns an empty sorted
+  set, as `Set.unionMany` does. `tryGetValue` returns a `voption`, `choose` takes a function that returns a `voption`,
+  and `partition` and `partitionWith` return struct tuples.
 * The functions of `SortedSet` that create a sorted set without a comparer, `builder`, `build`, `unionMany` of no sets,
   `map`, `choose`, `partitionWith` and `sortedSet { }` order with F# generic comparison, as `Set` does, where the BCL
   orders with `Comparer<'T>.Default`. A sorted set thus enumerates its elements in the order of the F# set of the same
@@ -116,12 +117,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `Seq.exists` and `Seq.forall` with `contains`, and the others take `SortedSet.ofSeq elements`, or
   `(SortedSet.forComparer (SortedSet.comparer set)).ofSeq elements` to give the elements the comparer of the first sorted
   set.
-* The element that stays when the comparer finds two elements equal follows the BCL: `addRange` keeps the element of the
-  sorted set, `union` keeps the element of the first sorted set, or of the larger one when both have the same comparer,
-  `intersect` keeps the elements of the second one, and the creation functions keep the first element of their input,
-  where `CreateRange` and `Create` with a span keep any of them. `isProperSuperset` is the exception: it counts the
-  elements of the second sorted set that the comparer of the first one finds equal once, as the documentation of
-  `IsProperSupersetOf` says, where the member counts each of them.
+* The element that stays when the comparer finds two elements equal follows the BCL: `union` keeps the element of the
+  first sorted set, or of the larger one when both have the same comparer, `intersect` keeps the elements of the second
+  one, and the creation functions keep the first element of their input, where `CreateRange` and `Create` with a span keep
+  any of them. When the second sorted set of `union` has another comparer and is much larger, the BCL refills the tree
+  from an unstable sort and either element can stay; only `addRange` keeps the element of the sorted set in every case.
+  `isProperSuperset` is the exception: it counts the elements of the second sorted set that the comparer of the first one
+  finds equal once, as the documentation of `IsProperSupersetOf` says, where the member counts each of them.
 * `SortedSet.minElement` and `maxElement` take the place of the properties `Min` and `Max`: they read the first and the
   last element in O(log n) and throw `ArgumentException` for an empty sorted set, as `Set.minElement` and
   `Set.maxElement` do, where the properties return `default`. `tryItem 0` and `tryItem (count - 1)` return a `voption`