What is the definition of a defendant's aider and abettor liability in a robbery case?

Generally, a defendant may be convicted of a crime either as a perpetrator or as an aider and abettor. ( 31.) "If the defendant himself commits the offense, he is guilty as a direct perpetrator. If he assists another, he is guilty as an aider and abettor." (People v. Perez (2005) 35 Cal.4th 1219, 1225.) An aider and abettor is a person who "'knowingly and with criminal intent aids, promotes, encourages, or instigates by act or advice'" a perpetrator of a crime. (People v. Beeman (1984) 35 Cal.3d 547, 560-561.) "For purposes of determining aider and abettor liability, the commission of a robbery continues until all acts constituting the offense have ceased." (People v. Cooper (1991) 53 Cal.3d 1158, 1164.)