How do I use an 'if' statment in an ocl expression for a derived attribute

What I'm trying to do is

if (self.ParentType.isNull) then

self.Name

else

self.ParentType.Name + ' - ' + self.Name

Thanks

Nick

I'm at a clients so I don't have your book to hand

Nick

if ParentType is an association then

  use ->isEmpty to check for null

else

  use ->IsNull 

endif