Suppose the taxpayer operates a retirement community. To obtain a unit in the community, each "tenant" must first pay a substantial entry fee. The fees are refundable if the tenant terminates the contract with the taxpayer, the amount of the refund determined by a schedule depending on how much time has elapsed since the tenant obtained the unit. Are the entry fees included in the taxpayer's gross income in full in the year received? If not, when should they be included? Explain with references to primary sources