public ISimpleSet <T> Diff(ISimpleSet <T> other)
{
var set = new SimpleHashSet <T>();
foreach (var item in this)
{
if (!other.Contains(item))
{
set.Add(item);
}
}
return(set);
}
public bool Contains(long studentId, long courseId) => Set.Contains(new CreditData(studentId, courseId));