AETNA INCEPTUS, CUJUS PERFECTIO PENDOT EX VOLUNTATE PARTIUM, REVOCARI POTEST SI AUTEM PENDET EX VOLUNTATE TORTIAE PERSONAE, VEL EX CONTINGENTI, REVOCARI NON POTEST

An act already begun, the completion of which depends on the will of the parties, may be revoked; but if it depend on the will of a third person, or on a contingency, it cannot be revoked. Bac Max. reg. 20.