听力原文： A federal law that requires people to supply their Social Security number when applying for a marriage license has forced thousands of couples around the country, particularly illegal immigrants, to put their wedding plans on hold. The law has been on the books for about a decade and was intended to make it easier to collect child sup- port payments. But in some places it has prevented even legal immigrants and some American citizens from getting married. Federal law requires states to record the Social Security numbers of all applicants for a professional license, driver's license, recreational license or marriage license. And Social Security numbers are not available to those who are in this country illegally or do not have permission to work. But whether and how the law is enforced varies dramatically from state to state, and even from county to county, with some authorities interpreting the law as saying that only those people who already have Social Security numbers need to supply them. But whether and how the law is enforced varies dramatically from state to state, and even from country to country, with some authorities interpreting the law as saying that only those people who already have Social Security numbers need to supply them. What is the purpose of the mentioned federal law?