Beginner question : complex cross field/attribute validation possible ?

 Dear all,

 i looked into the XML Schema specs, but i am not sure if a validation rule
such as

 -  for instance : if field a==21 and b==32 and field x==field z then c must
be not null)

 can be expressed in XML Schema ?

 If yes,

 - what XML Schema version spec has to be supported ?
 - Any validation engines that do that already ?

 Any other XML standard with which I can do that ?

 Thank you
 Frank