What is the test for revocation of probation for failing to pay restitution?

Because the issues of ability and willful failure to pay relate to the factual determination whether defendant violated her probation, rather than to whether, in the trial court's discretion, the interests of justice require revocation, I am unpersuaded by the dicta of People v. Ryan (1988) 203 Cal.App.3d 189, 199, 249 Cal.Rptr. 750, which the majority cite for the proposition that the trial court must make an express finding that a probationer has the ability to pay restitution before it can revoke probation for the failure to pay same.