XML data
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... stanza that contains a child
element it does not understand, it SHOULD ignore that specific XML
data, i.e., it SHOULD not process it or present it to a user or
associated application (if any). In particular:
...
... entity receives a message or presence stanza that
contains XML data qualified by a namespace it does not
understand, the portion of the stanza ...
... contains a child element it does not understand, it SHOULD ignore
the associated XML data by passing it on untouched to the
recipient.
...