What is the test for determining whether a defendant is mentally competent?

^9. "Penal Code section 1369 sets forth the procedures for the trial in which the question of the mental competence of the defendant is to be determined. It provides that such a trial may be 'by court or jury' ([] 1369), and that, if by jury, it must be decided by unanimous verdict ([] 1369, subd. (f)). It also provides for the appointment by the court of one or more experts to examine the defendant ([] 1369, subd. (a)), the introduction of evidence by the defendant and the People ([] 1369, subds. (b), (c) & (d)), and the presentation of argument by each thereafter ([] 1369, subd. (e)). It states that '[i]t shall be presumed that the defendant is mentally competent unless it is proved by a preponderance of the evidence that the defendant is mentally incompetent.' ([] 1369, subd. (f).)" (People v. Rells, supra, 22 Cal.4th at p. 866.)

^10. (In re Tahl (1969) 1 Cal.3d 122.)