Add / delete instances to a multiple value (SOLVED)

How to add (or delete) an instance (via business rule) to a multiple allowed value (list) of an attribute ?

Example :
When a Person has a multiple allowed attribute : Department
How to add the 2nd department ?
Or just delete a department of the "list"

Something like :
Person.ob_Department = Person.ob_Department + ThisDepartment

INSERT Department IN Person.ob_Department

I generally do it a little differently depending on if the Reference is Single or Multiple..

So.... in the situation above, INSERT Department IN Person.ob_Department, ob_Department is Singular so I would ALWAYS use..

Person.ob_Department = Department

However, if the Departments Reference is multiple... ie... a Person can belong to many Departments....

Person.pm_Departments for example... then I would use

INSERT Department IN Person.pm_Departments

Thnx!