Document Stratego gradual types change in nightly

source/release/note/vnext.rst

@@ -8,3 +8,10 @@ See the corresponding :ref:`migration guide <vnext-migration-guide>` for migrati
 
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+Stratego
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+Stratego has two new reserved words: ``cast`` and ``is``. Local variables can be reserved words is they start with ``'``, so you can use ``'cast`` and ``'is``. 
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+Under the Stratego language options in your ``metaborg.yaml`` file you can turn on the gradual type system, if you use the incremental compiler. This option is ``gradual: static``, and only tests the types statically. The default is ``gradual: none`` right now, meaning the gradual type system is not on by default. There is an experimental third option ``gradual: dynamic`` which not only checks the types statically but also inserts casts to check types dynamically where necessary.