Marks a key removed from what a lazy for ... from template supplies -- -key in the loop body, or a list edit of a key the template turns out not to have. Only ever appears inside a Cooper.Merge.Layered's overrides; resolving the Layered drops the key.

Before it existed, a -key in such a loop body was a no-op: the key lives in the template, not in the tree the delete looked in.