What is the test for determining liability as an aider and abettor of a robbery?

Section 31 provides in relevant part that all persons concerned in the commission of a crime, or who aid and abet in its commission, are principals in any crime so committed. To prove liability as a principal, the prosecution must show an aider and abettor intended to facilitate or encourage the principal offense prior to or during its commission. (People v. Cooper (1991) 53 Cal. 3d 1158, 1160.) For purposes of determining liability as an aider and abettor, the commission of robbery continues so long as the loot is being carried away to a place of temporary safety. (Id. at pp. 1169-1170.)