From language/subscripts

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multi method DELETE-POS (::?CLASS:D: $index)

Expected to delete the element at position $index, and return the value it had. This is what postcircumfix [ ] calls when invoked like @foo[42]:delete.

What "deleting" an element means, is up to your type.

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.