The issue was that some code expected the "extend" sockets
to always exist. This was already always true from the user
point of view, but not internally.
There are different possible fixes, but this patch makes sure
that the extend socket will be available as expected. This is
achieved by making them part of the static node declaration.
We want to extend the static declaration of such dynamic
nodes anyway, to improve reflection capabilities without
having to instantiate nodes.
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