What is the test for dismissing a prior strike conviction for sentencing purposes?

Under section 1385, subdivision (a), a trial court may dismiss a prior strike conviction for sentencing purposes if the defendant falls outside the spirit of the three strikes law. (People v. Williams (1998) 17 Cal.4th 148, 161.) When doing so, the trial court "must consider whether, in light of the nature and circumstances of [the defendant's] present felonies and prior serious and/or violent felony convictions, and the particulars of his background, character, and prospects, the defendant may be deemed outside the scheme's spirit, in whole or in part, and hence should be treated as though he had not previously been convicted of one or more serious and/or violent felonies." (Ibid.)