Definitions in structures (modules) are extended as follows:
Toplevel definition in structure can of course be join-definition. The additional def exception exception constructs enables matching by structure in try … with …handlers. The construct, somehow aleviates the burden of having generative exceptions in a distributed context. The name exception must be the name of a pre-existing exception, and def exception exception must be executed at most once. See Section 2.3.2 for an example.