Message
Undeclared foo prefix: bar
Explanation
All namespace prefixes must be declared prior to use. Namespace prefixes are case-sensitive.
Solution
Correct the typo, or declare the prefix with an xmlns:
attribute, for example:
<Url type="application/rss+xml"
xmlns:example="http://example.com/opensearchextensions/1.0/"
template="http://example.com?f={example:format?}"/>
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