The map-lookup-forgiveness problem

At today's meeting Michael Kay asked for a detailed use-case for map
returning a map with no keys as a result of a failed lookup.

Here is this described in https://github.com/qt4cg/qtspecs/issues/105  "[XPath]
Proposal: Maps with Infinite Number of Keys: Total Maps and Decorated maps
#105"


Using this representation of the empty sequence, we can provide a solution
for the *"Forgiveness problem"
<https://xmlcom.slack.com/archives/C011NLXE4DU/p1616167871037100>* raised
by Jarno Jelovirta in the XML.Com #general channel in *March 2021*:

This expression will raise an error:

[map {"k0": 1}, map{"k0": [1, 2, 3]}]?*?("k0")?*

*[XPTY0004] Input of lookup operator must be map or array: 1.*

To prevent ("forgive", thus "Forgiveness Problem") the raising of such
errors we could accept the rule that in XPath 4.0 any expression that
evaluates to something different than a map or an array, could be coerced
to the following map, which returns the empty sequence as the corresponding
value for any key requested in a lookup:

map {'\' : ()
}  (: produces the empty sequence  for any lookup:)


Thanks,
Dimitre