// Implement an exclusive OR function called `preferredName` that has the following interface:
var FirstName,
LastName;
preferredName(FirstName, LastName);
// -> false
FirstName = 'Hank';
preferredName(FirstName, LastName);
// -> 'Hank'
LastName = 'Yates';
preferredName(FirstName, LastName);
// -> false
FirstName = '';
preferredName(FirstName, LastName);
// -> 'Yates'