Hi alasdairhitchen! At the moment, you can’t use includes/excludes (“relationship-mode”) collections as sub-expressions in a pattern-based collection. In your example you’d have to convert foos to a path expression, e.g.
uniform pathExpression collection:foos:membershipExpression = "/root/foo*"
(or even “/root/foo1 + /root/foo2”) to reference it in foos_pbc.
There’s also a new method on CollectionMembershipQuery that computes an equivalent path expression from a rule map that should be available in the 24.08 release.