who, whom, whose: case and the lemma who — proposal

Proposal: accept ud-next (who, whom, whose: case and the lemma who (English)).

What exactly in mova.

lemma who for whom, whoever for whomever;
who/whoever: Case=Nom or Case=Acc by syntactic function (rules mova.ud-next.who-* in Converter mova ⇄ ud-next); whom/whomever — always Acc, because the form wins;
whose with a noun (nmod:poss) — Case=Gen|Poss=Yes; standalone — Poss=Yes.

Why.

More informative. Case shows the role of who in a relative clause or question: the man who I saw (Acc) versus the man who saw me (Nom). For annot and RAG this is the role of the "gap" in the relative clause, one of the most valuable clause-level features (the UD survey digest «future», section 4, item 4).
More consistent.
- Personal pronouns in EWT already have Case by position (you Nom 2096 / Acc 651).
- Who pronouns are the only pronouns with case but without Case.
- GUM, GENTLE, LinES and ParTUT already write the lemma who for whom; EWT, PUD and LittlePrince do not.
Lossless into the 2.18 standard. The reverse rules are deterministic (ud-next.who-case-drop, ud-next.whose-case-drop, ud-next.whom-lemma): drop Case and restore the lemma from the form. A round trip of EWT 2.18 gives identity: 24 lemmas, 460 features.
The registry already allows PRON Case=Nom|Acc|Gen and Poss=Yes. Export to 2.18 and to ud-next is valid.

Evidence from the data (EWT 2.18, engine):

whom with lemma whom — 23, whomever — 1;
who pronouns without Case — 447 of 447;
whose without Gen — 13, all with a noun;
expected values after the rules: Acc 37, Nom 410, Gen 13 (Converter mova ⇄ ud-next).

Risks.

Case for who is taken from the basic function, but in 2 free relatives and in one predicative case — from the position inside the subordinate clause (edge cases are described in convert.md).
Whether EWT 2.19 will do this is not visible from the data (there is no dev in the snapshot). If it does not, export to "EWT 2.19" will go through the same reverse rules.

Check.

Gates udnext.whom-who-acc, udnext.who-case, udnext.whose-dependent (error);
hints udnext.who-object-acc, udnext.who-subject-nom, udnext.whose-independent (warn);
the existing en.morph.whom-acc (Interrogative and relative pronouns: who, whom, whose, which, what, that).

Origin. who, whom, whose: case and the lemma who (English), The pronouns who, whom, whose in 2.18: TODO in the guideline, #517.

Mova decides.