What is the test for aiding and abetting an armed robbery?

"An essential element of armed bank robbery ... is that the principal was armed and used the weapon to jeopardize [lives]." United States v. Short, 493 F.2d 1170, 1172 (9th Cir.), cert. denied, 419 U.S. 1000 (1974). In order to find a defendant guilty of aiding and abetting an armed robbery, the evidence must establish that the defendant knew the principal was armed and intended to use the weapon. Id.