What is the test for a motion for a defendant to be treated as if he had not previously been convicted of serious and/or violent felonies?

In ruling on a Romero motion, the 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 (1998) 17 Cal.4th 148, 161.)