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The text of this query is fn:avg(( (xs:float("NaN"), xs:decimal("-999999999999999999") , "a", (), "3") )) I believe that this can validly return "NaN", under the criteria for "errors and optimization": once you've established that there is only one possible non-error result (NaN in this case) you are allowed to exit early and return that result without evaluating more of the sequence to look for errors.
Also applies to fn-avg-mix-args-056
Added additional output for fn-avg-mixed-args-047 and 056 with expected output as "NaN"