3 | Use XML syntax only. If a tag got both container and
non-container callbacks, the non-container callback is called
when the empty element form (i.e. the one ending with
'/>' ) is used, and the container callback otherwise.
If only a container callback exists, it gets the empty string
as content when there's none to be parsed. If only a
non-container callback exists, it will be called (without the
content argument) for both kinds of tags.
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