These exception types are intended as a hook for a programming
environment to recommend packages that can provide a module that isn't
found (through an uncaught-exception handler). The new exceptions are
generated by the default module name resolver and defaultload
handler. In addition to the exceptions, there's a new
`prop:exn:missing-module' property and `exn:missing-module?', which is
what an environment should use to detect relevant exceptions.
There's also a new `current-module-path-for-load' parameter, which
just provides a commuincation path from the module name resolver to
the the load handler. The parameter is a relatively ugly piece of the
puzzle, but it's the best I could find to squash the new functionality
into the existing stack of handlers.
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