uses f[key] as the value associated with a missing key.
- KeyUnion returns a list of associations in which all the associations have keys ordered in the same way.
- If f is an association containing all the keys to be returned by KeyUnion, then the order of keys in f will determine the order of keys in the associations returned.
- In other cases, the keys will appear in order of their first occurrence in the sequence of associ.
- KeyUnion can be used not only on Association objects, but also on lists of rules.
Introduced in 2014