key and keyref to express dependency

hi everybody,
  i really don't find the W3C SChema specification clear, and have a question:
can we express in the W3C Schema that the presence of an element A depends
en the presence of an element B?
i thought about key and keyref elements,but i doubt they just impose
thank you 

Received on Wednesday, 5 June 2002 16:55:09 UTC