The only rule in RegularUser I could find is
If EXISTS SystemUser WHERE (SystemUser.LoginName=ThisRegularUser.LoginName AND SystemUser.ID<>ThisRegularUser.ID) Then
REPORT ERROR 'User with this login name already exists. Please choose a different name'
And the rule worked for my case by now is:
If NOT(Appointment IS NEW ) AND Appointment.Stage1 IS DEFINED Then PROTECT ThisAppointment.Stage1 FROM ALL EXCEPT System
Now testing it.[/code]