The Kin Score, Facet By Facet: Create-or-Update Ergonomics

This is part seven of nine, one Kin Score facet each business day. Yesterday I covered Operational Transparency. Today is Create-or-Update Ergonomics, and it asks one question: if you accept writes, can a caller create-or-update a record in one call, keyed on an identifier they already hold, and find out afterwards which of the two actually happened?