What is the test for awarding attorney fees under section 1021.5 of the California Civil Procedure Act?

"The trial court, in considering fee awards to private litigants on the facts and record applicable to each particular case, must carefully walk the line between unreasonably transmuting section 1021.5 into an unwarranted cornucopia of attorney fees for those who intervene in, or initiate litigation against, private parties under the guise of benefiting the public interest while actually performing duplicative, unnecessary, and valueless services, and providing appropriate compensation under that statute in cases where the colitigating private party does render necessary, significant services of value and benefit to the public." (Committee to Defend Reproductive Rights v. A Free Pregnancy Center, supra, 229 Cal.App.3d at pp. 643-644, 280 Cal.Rptr. 329.) The instant case is an example of the latter situation.