`undef` is recognized, `UNDEF` is not

[VladimirAlexiev]

This script is parsed ok:

declare function local:rdf_property ($property, $id as xs:string) {
  let $propName := $property/name()
  let $value    := $property/text()
  for $prop $dt where {
    values ($propName1                    $prop                                $dt           ) {
      ("GF_Ausn"                          "xp:floorAreaExcluded"               "xsd:decimal" )
      ("nutzungsform"                     "xp:formOfUse"                       undef         )
    }
    filter ($propName1 = $propName)
  }
  construct {<{$id}> <{$prop}> {$value}} # ^^{$dt} 
};

()

But if I replace undef with UNDEF, I get this error:

line 4:12 no viable alternative at input '$dt'
line 5:4 mismatched input 'values' expecting RCURLY

• The problem is that SPARQL is case-insensitive so UNDEF should also be accepted
• Another problem is that the error message is totally unhelpful: the location is two levels up, and then some nodes away. I spent 2h debugging this (thinking that maybe it doesn't allow a second var in for. The xsparql parser is terrible that way :-(