What is the test for claiming that a plaintiff's substantial rights was forfeited by the failure of trial counsel to object?

Without addressing whether the claimed error affects defendant's substantial rights, the People claim the instructional error was forfeited by the failure of trial counsel to object. ( 1259; People v. Andersen (1994) 26 Cal.App.4th 1241, 1249 ["Ascertaining whether claimed instructional error affected the substantial rights of the defendant necessarily requires an examination of the merits of the claimat least to the extent of ascertaining whether the asserted error would result in prejudice if error it was"].) We disagree.

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