DOCBOOK: ISO Entities

Hi,

how can implement symbols such as....

      "DOES NOT CONTAIN AS NORMAL SUBGROUP"  <!-- iso-amsn seems to be
responsible -->

I know the Identifier to use is .... 

      PUBLIC "ISO 8879:1986//ENTITIES Added Math Symbols: Negated
Relations//EN//XML"

What do I have to do with it?

Thanks for your help,
Chris