LowerToBuilder lowers each observer in obs to a corpus builder observer.

OnCompileStart sends a compile start message to every observer in obs, containing the quantity set qs.

OnMachineNodeAction distributes m to every observer in obs.

OnRunStart sends a run start message to every observer in obs, containing the quantity set qs.

KindCompileStart means that the machine node is starting its compile phase.

KindRunStart means that the machine node is starting its run phase.

Message is the type of messages from a machine node observer.

Observer is the interface of anything that observes machine node behaviour.

MessageKind is the enumeration of possible messages from a machine node observer.