[BGL] Installing a property

Hi I have a mass spring system where the particles are mapped to the vertex name property of the vertex. Now I've read (in the BGL red book) that one can install a custom property using an enumerated type. However it says that the enum must be a unique number. How do I enforce this in my code? Do I have to manually track the numbers myself? Thanks sashan [Non-text portions of this message have been removed]