A module defining nodes could not be run directly: the decorator refused
`__main__` while the module body was still executing, so a `if __name__ ==
"__main__"` self-check beside the nodes was impossible and the checks had
to live in a separate pytest file. The refusal now fires where the
generated body is written — document() and shims(), both, since sync writes
the document first — and the decorator hands the function back as it always
did. Syncing a __main__-defined node is still refused, with the same words.
stroblme
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