What is the test for giving jury instructions that single out one witness?

Appellant analogizes from the principle that it is impermissible for the court to give the jury instructions that single out one witness. "When the proposed instruction focuses exclusively or primarily on the testimony of one witness, it runs afoul of a well settled corollary of the foregoing rule, i.e., that it is ' "improper for the court to single out a particular witness and to charge the jury how his evidence should be considered." ' (People v. Lyons (1958) 50 Cal.2d 245, 271 [324 P.2d 556].)" (People v. Wright (1988) 45 Cal.3d 1126, 1135, fn. 6, 248 Cal.Rptr. 600, 755 P.2d 1049.)