XQuery: Optimizations for grouping single values

[ChristianGruen]

for $b in ('a-1', 'a-2')
group by $c := substring-before($b, '-')
return $c

…is simplified to "a".

The same should happen for single items. The following expression …

for $b in ('a-1')
group by $c := substring-before($b, '-')
return $c

…is simplified to group by $c := "a" return $c (illegal as input query, but valid as compiled query).