Proposal to add fallback behaviour to map:get and array:get - #289

         <ulist diff="add" at="2022-12-16">
            <item><p>It might return a conventional value such as <code>NaN</code> to indicate that no matching
            key was found.</p></item>
            <item><p>It might raise a dynamic error, by means of a call on <code>fn:error</code>.</p></item>
            <item><p>It might compute a result algorithmically. For example, if the map holds a table of
            abbreviations, such as <code>map{'CA':'Canada', 'UK':'United Kingdom', 'US':'United States'}</code>,
            then specifying <code>fallback := fn:identity#1</code> has the effect that the key value is returned
            unchanged if it is not found in the map.</p></item>
         </ulist>