What is the test for a principal to be convicted of a crime?

Since 1872, the term "principal" has been defined in California statutory law, as relevant here, as including "[a]ll persons concerned in the commission of a crime . . . whether they directly commit the act constituting the offense, or aid and abet in its commission . . . ." ( 31.) California statutory law has also stated, since 1872 as well, "The parties to crimes are classified as: [] 1. Principals; and, [] 2. Accessories [after the fact]." ( 30; see also 32; People v. Talbott (1944) 65 Cal.App.2d 654, 660-661 ["A defendant in a criminal action therefore is convicted as a principal or accessory or not at all; and in this connection, the fact that principals may be conspirators is immaterial."].)