Support for proper unary predicates

[ThomasHaas]

This issue is just a reminder that we should eventually support dynamic tagging (= arbitrary unary predicates) and not just static tagging (= our current filters).

[hernanponcedeleon]

Once we have that, here we have a test case

[hernanponcedeleon]

Related to #254

[ThomasHaas]

Added in #908