Procne (Πρόκνη) is the Greek name of the Athenian princess transformed into a swallow (or in some versions, the nightingale) — sister of Philomela. Her tragic story of family vengeance is told in Ovid's Metamorphoses.
Procne and Philomela are inseparable in classical literature.
Procne reduces to one — the number of Greek swallow.