What is the test for dismissing a prior strike allegation?

In deciding whether to dismiss a prior strike allegation, a trial court "must consider whether, in light of the nature and circumstances of his 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." (People v. Williams, supra, 17 Cal.4th at p. 161.)