The previous behaviour of `stop` was inappropriate: it always stopped
`Syndicate.Turn.activeFacet`, which is an instance of dynamic binding.
Now, it instead stops the unique lexically-innermost lexically-apparent
facet - the special name `currentSyndicateFacet` - by default. It is an
error if no such facet is lexically apparent (if `currentSyndicateFacet` is
unbound). This makes it similar to `break` and `continue` in structured
programming.
In addition, an expression denoting a facet can now be used with `stop` to
override this default - again, like `break` and `continue` in some
languages.
Finally, `react` can now be preceded by a label, which binds the label as a
variable denoting the newly-created facet (inside the facet's scope).
Tony Garnock-Jones