The United Auto Workers (UAW) represents workers at Caterpillar, Inc., and negotiates labor contracts on their behalf. A 1988 labor agreement provided lifetime no-cost medical benefits for retirees but did not state when the employees’ rights to those benefits vested. This agreement expired in 1991. Caterpillar and the UAW did not reach a new agreement until 1998. Under the new agreement, retiree medical benefits were subject to certain limits, and retirees were to be responsible for paying some of the costs. Workers who retired during the period when no agreement was in force filed a suit in a federal district court to obtain benefits under the 1988 agreement. Review the Employee Retirement Income Security Act vesting rules for private pension plans. What is the most plausible application of those rules by analogy to these facts? Discuss.