For future readers:
Hillegass tells you to add 2 methods: -insertObject:inEmployeesAtIndex:
and -removeObjectFromEmployeesAtIndex:
. In the second method you have to do:
NSUndoManager *undo = [self undoManager];
[[undo prepareWithInvocationTarget:self] insertObject:p inEmployeesAtIndex:index];
But I did:
NSUndoManager *undo = [self undoManager];
[[undo prepareWithInvocationTarget:self] insertObject:p atIndex:index]; //atIndex: instead of inEmployeesAtIndex: