Since the native syntax requires that attribute names and block types
be identifiers, there is no way for "//" to be a valid symbol in a body.
Therefore we can special-case it here as an ignored property name that
users can then use for comments.
This special handling intentionally does not apply to objects representing
zcl object expressions, since it _is_ valid to have non-identifier keys
there and so "//" may be a legitimate object key for some applications.
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