You can still write x=_|_ in ML, you just have to be subtler about it.

True, but the question still stands, since you can still test it by just telling the kernel to insert a (fake) leap second.

It also should not be that hard to provide your own upstream ntp server, and have that generate leap seconds at will. Both machines could be VMs, too.

If we are talking Common Lisp, the default test is actually EQL. ((eq 1 1) is not necessarily T).