A protocol message that was structurally dead code
The protocol sent an Opened receipt 'only if the sender asked for it', but no field existed to express the request, so the message could never be sent.
Built, and nothing used it
Shipped, typechecked, dependency-injected, unreachable. Nothing ever called it.
The protocol says an Opened receipt is sent only if the sender asked for it. There was no field anywhere to express that request, so nothing could ever ask, so the message could never be sent. Added notify_on_open to Offer itself rather than inventing a separate control message, since it's a property of the offer.
Worth noticing as a category: a spec sentence with a conditional in it ("only if X") is a place to check that X is representable at all.