What is the test for exclusion of a witness?

Under Evidence Code section 777, a trial court may generally "exclude from the courtroom any [nonparty] witness not at the time under examination so that such witness cannot hear the testimony of other witnesses." (Evid. Code, 777, subd. (a); see 777, subd. (b).) "The purpose of [a witness exclusion] order is to prevent tailored testimony and aid in the detection of less than candid testimony. (Geders v. United States (1976) 425 U.S. 80, 87.)" (People v. Valdez (1986) 177 Cal.App.3d 680, 687.) "It is . . . anticipated trial counsel will advise witnesses to remain outside until they have testified and are excused." (Id. at p. 692.)