+ ;; FIXME: Relying on POSITION allowing both :TEST and :TEST-NOT
+ ;; arguments simultaneously is a little fragile, since ANSI says
+ ;; we can't depend on it, so we need to remember to keep that
+ ;; extension in our implementation. It'd probably be better to
+ ;; rewrite this to avoid passing both (as
+ ;; LIST-REMOVE-DUPLICATES* was rewritten ca. sbcl-0.7.12.18).