Two test cases: data, queries, results and manifest.ttl attached.
with UNSAID and also with OPTIONAL.
An example like this has just been asked on jena-dev.
"Find all the things that are not of a given class."
type :foo (Q2) and take all ?x in Q2 out of Q1).
"find all the people with exactly one known email address"
of the match with dawg:bound.
See MySQL manual "13.1.8.3 Subqueries with ANY, IN, and SOME"
for :email twice.  Adding a third email for :p1 gets 6 rows etc etc.
It could be argued that this is where the cost of a subquery has gone.
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