What is the test for determining pension eligibility standards in the context of a trustee's fiduciary obligation?

"Courts are extremely reluctant to substitute their judgments for the judgments of trustees and will do so only if the actions of the trustees are not grounded on any reasonable basis." Ponce v. Construction Laborers Pension Trust, 628 F.2d 537, 542 (9th Cir.1980) (Ponce I ). Short of "plainly unjust" standards, trustees have wide latitude in formulating pension eligibility rules. Id. at 544. "A court should interfere only when the rule is unreasonable or its enforcement arbitrary." Giler v. Board of Trustees, 509 F.2d 848, 849 (9th Cir.1974).