Is a person who aids and abets the commission of a crime considered a principal in the crime?

"A person who aids and abets the commission of a criminal offense is considered a principal in the crime. [Citation.] In order for criminal liability to be imposed under an aiding and abetting theory, the person must 'act with knowledge of the criminal purpose of the perpetrator and with an intent or purpose either of committing, or of encouraging or facilitating commission of, the offense.' " (People v. Swanson-Birabent (2003) 114 Cal.App.4th 733, 740.) However, "[m]ere presence at the scene of a crime which does not itself assist its commission or mere knowledge that a crime is being committed and the failure to prevent it does not amount to aiding and abetting." (In re

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