What is the test for murder in the first degree?

'Homicide committed in the perpetration of robbery is murder in the first degree regardless of whether the killing be wilful, deliberate and premeditated or even accidental. [Citations.] A principal in a robbery resulting in a homicide may be found guilty of murder in the first degree notwithstanding that his co-defendant fired the fatal shot [citation], and principals include those persons who advise or encourage the commission of a crime or who aid and abet in its commission. [Citations.]' (People v. Machado, 150 Cal.App.2d 190, 193-194, 309 P.2d 903, 905.)