in Sources/_CollectionsTestSupport/MinimalTypes/_CollectionState.swift [157:171]
func insert(count: Int, at offset: Int) {
precondition(count >= 0 && offset >= 0 && offset <= self.count)
guard count > 0 else { return }
self.count += count
switch strategy {
case .allIndices:
// Do nothing
break
case .afterChange:
let rest = offset ..< indexValidity.count
self.indexValidity.replaceSubrange(rest, with: repeatElement(id, count: rest.count + count))
self.endIndexValidity = self.id
assert(indexValidity.count == self.count)
}
}