What is the defence of self-defense against tear gas?

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Notes:

^1. The trial court did not instruct the jury that duress was a defense to the charge of use of tear gas, and defendant does not claim this was error. The court instructed the jury that it could consider the defense of self-defense for the crime of use of tear gas and a tear gas weapon.

^2. Defendant's trial counsel also objected on the ground of hearsay. Defendant does not pursue this argument on appeal. An expert witness may generally base his or her opinion on hearsay. (Evid. Code, 801, subd. (b); People v. Gardeley (1996) 14 Cal.4th 605, 618.)

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