Set implements a collection of unordered values with no duplicates.

Arrays are not sets. Trying to treat them as if they are is an error, and will create subtle problems. What should be the result of the following operations? [1, 1] | [1] [1] | [1, 1] Of course, there are more interesting examples. These two are to get you started. I don't care what the results currently are. I don't care what you think they should be. I can present extremely strong arguments for various answers. For this reason, I believe that #| is an ill-defined concept. Generalizing an ill-defined concept is a world of pain. If you insist on treating objects of one class as if they were members of a different class, there should be bumps in the road to at least warn you that maybe this is a bad idea. I'm not going to argue that we should remove or deprecate #|. I don't think of myself as a fanatic. But encouraging this sort of abuse of the type system just creates problems.