What is "substantial evidence" that raises a reasonable doubt about a defendant's competence to stand trial?

As we have stated: "Pate v. Robinson [ (1966) 383 U.S. 375, 86 S.Ct. 836, 15 L.Ed.2d 815] stands for the proposition that an accused has a constitutional right to a hearing on present sanity if he comes forward with substantial evidence that he is incapable, because of mental illness, of understanding the nature of the proceedings against him or of assisting in his defense. Once such substantial evidence appears, a doubt as to the sanity of the accused exists, no matter how persuasive other evidence--testimony of prosecution witnesses or the court's own observations of the accused--may be to the contrary." (People v. Pennington (1967) 66 Cal.2d 508, 518, 58 Cal.Rptr. 374, 426 P.2d 942.) "Substantial evidence is evidence that raises a reasonable doubt about the defendant's competence to stand trial." (People v. Frye (1998) 18 Cal.4th 894, 952, 77 Cal.Rptr.2d 25, 959 P.2d 183, italics added.)