RefusesMerge

A stat whose Stat.merge always throws, declared so a container can find out before it starts.

A group merges its children in declaration order with no way to undo one, so a child that throws partway leaves the group permanently inconsistent: the entries before it carry both shards and the entries after it carry one. Declaring the refusal lets the group check every child first and throw without touching any of them, and lets the message name the offending key instead of surfacing from whichever stat happened to be reached.

Inheritors