What is ACTUS INCEPTUS, CUJUS PERFECTIO PEN- DET EX VOLUNTATE PARTIUM, REVOCARI POTEST; SI AUTEM PENDET EX VOLUNTATE TER- TISE PERSONSE, VEL EX CONTINGENT!, 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.