What is the test for determining whether a witness is an accomplice under section 1111 of the Penal Code?

" ' "[W]henever the testimony given upon the trial is sufficient to warrant the conclusion upon the part of the jury that a witness implicating a defendant was an accomplice,' " the trial court must instruct the jury, sua sponte, to determine whether the witness was an accomplice. [Citation.] If the testimony establishes that the witness was an accomplice as a matter of law, the jury must be so instructed. [Citation.] In either case, the trial court also must instruct the jury, sua sponte, '(1) that the testimony of the accomplice witness is to be viewed with distrust [citations], and (2) that the defendant cannot be convicted on the basis of the accomplice's testimony unless it is corroborated ....' [Citation.]" (People v. Zapien (1993) 4 Cal.4th 929, 982.)

"Whether a person is an accomplice within the meaning of [Penal Code] section 1111 presents a factual question for the jury 'unless the evidence permits only a single inference.' [Citation.] Thus, a court can decide as a matter of law whether a witness is or is not an accomplice only when the facts regarding the witness's criminal culpability are 'clear and undisputed.' [Citations.]" (People v. Williams (1997) 16 Cal.4th 635, 679.)

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