From language/subscripts

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multi method DELETE-KEY (::?CLASS:D: $key)

Expected to delete the element associated with $key, and return the value it had. This is what postcircumfix { } calls when invoked like %foo<aa>:delete.

What "deleting" an element means, is up to your type - though it should usually cause EXISTS-KEY to become False for that key.

Implementing this method is optional; if you don't, users trying to delete elements from an object of this type will get an appropriate error message.