A +key/-key list edit (CASC.md §8.4) whose base or operand is not known until resolution -- +tags = @{more}, an edit of a list that is itself still a reference, or an edit of a key a for ... from template supplies. Cooper.Merge leaves one at the path; Cooper.Resolver resolves both sides and applies the edit element-wise.

Applying the edit at merge time instead, as was once done, appended an unresolved reference as one element (["a", ["b", "c"]] for tags = ["a"] then +tags = @{more}) and ignored a template's list entirely.

base is a Cooper.Ref.Config defaulting to Cooper.Merge.Absent when it reads from a for ... from template, so a template without the key is distinguishable from one whose key is nil. path is only for error messages.