Currently, adders and removers must be prefixed with "add" and "remove". When applying DDD, this sometimes doesn't make sense, for example:

class Contact
{
    public function addGroup(ContactGroup $group) { }
    public function removeGroup(ContactGroup $group) { }
}

Here it would make much more sense to prefix the methods with "join" and "leave":

class Contact
{
    public function joinGroup(ContactGroup $group) { }
    public function leaveGroup(ContactGroup $group) { }
}

The PropertyAccessor should provide a way to use these methods.