 1940).In "Keyes", the Supreme Court held that if "school authorities have been found to have practiced purposeful segregation in part of a school system", the burden of persuasion shifts to the school to prove that it did not engage in such discrimination in other segregated schools in the same system.In "Director, Office of Workers' Compensation Programs v. Greenwich Collieries", the Supreme Court explained that "burden of proof" is ambiguous because it has historically referred to two distinct burdens