Ickle purely negative fulltext subqueries cause empty results

When using parethesis in an Ickle full text predicate, it can generate subqueries that are pure negative, e.g.

from IspnEvent where (tags : ('tagA' and (not 'a0')))

In Lucene, the query
+tags:taga +(-tags:a0)

is different from
+tags:taga -tags:a0

The latter works as expected, but the former brings empty results since Lucene does no support purely negative subqueries. In order for the former query to work, it needs to add all documents as another term to the subquery, for e.g. +tags:taga +(*:* -tags:a0).