The Mayo case involved a substantially revised, prospective regulation which imposed an irrebuttable presumption that a medical resident working 40 hours or more is not a student regularly attending classes and, therefore, does not qualify for an exemption from FICA taxes. There is ample legislative history supporting this regulation - which was utilized by the Eighth Circuit, but not by the Supreme Court. The unanimous Supreme Court applied Chevron and determined the regulation was not arbitrary or capricious. Mayo would suggest the pro-taxpayer direction of these six changes and, in itself, would support the taxpayer since, in determining the reasonableness of the applicable regulations in Mayo, the Court pointed to an anti-medical school resident provision in a related section, noting that such a choice casts doubt that Congress intended to insulate medical residents from FICA's reach.