/// <summary>
/// Returns a NameGender value indicating the likely gender of the Pronoun based on its
/// referent if known, or else its PronounKind.
/// </summary>
/// <param name="referencer">The Pronoun whose gender to lookup.</param>
/// <returns>A NameGender value indicating the likely gender of the Pronoun.</returns>
private static Gender GetGender(IReferencer referencer) => referencer.Match()
.Case((PronounPhrase p) => DeterminePronounPhraseGender(p))
.When(referencer.RefersTo != null)
.Then((from referrenced in referencer.RefersTo
let gender = referrenced.Match()
.Case((NounPhrase n) => DetermineNounPhraseGender(n))
.Case((Pronoun r) => r.Gender)
.Case((ProperSingularNoun r) => r.Gender)
.Case((CommonNoun n) => Gender.Neutral)
.Result()
group gender by gender into byGender
where byGender.Count() == referencer.RefersTo.Count()
select byGender.Key).FirstOrDefault())
.Case((ISimpleGendered p) => p.Gender)
.Result();
