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Which features of XPath 3.0 are optional features? * Dynamic function call? * Inline function expressions? Are there other candidates for optional features?
I think the only reasonable candidate for an optional feature is "higher order functions" which would include the function() sequenceType syntax, inline functions, function literals (f#n), partial application, dynamic callling, functions such as fn:map, and functions such as fn:function-name and fn:function-lookup. I have an open mind as to whether making this an optional feature would be a good idea. I think that before doing so, we would want to see a reasoned argument from an implementor or from the developers of a significant host language (e.g. XForms) as to why the feature is unwanted.
Conformance to XPath is specified in the document defining the host language (e.g., XQuery), so it's not necessary to discuss such conformance issues in the XPath spec itself.