{extra-default (fn/fnull :+ 0 0 0 0)
 missing (fn/fnull)
 missing-call-argument ((fn/fnull :+ 0))
 missing-second-argument ((fn/fnull :+ 0 0) null)
 missing-second-of-three ((fn/fnull :+ 0 0 0) null)
 no-default (fn/fnull :fn/identity)}