Tax question.

After a 20 year marriage, divorce occurred 1990 with me receiving lifetime alimony and I paying the tax on the alimony per the law current in 1990. There have been no modifications to the taxing of the alimony. Annually I pay federal and state tax on the alimony. The law governing this 1990 court adjudication says I can contribute up to annual total of the alimony taxed to contribute to a Roth IRA. AARP has been doing my taxes since age 50. They would like validation that I am legally allowed to contribute to the Roth based on taxed alimony. The law has changed for divorces adjudicated subsequently but as I read the IRS tax code and Roth contribution code, I am eligible lo do this based on my 1990 decree. Please verify.