xacml message

Subject: why minoccurs != 0 for VariableDefinition elements?

From: Argyn <jawabean@gmail.com>
To: xacml@lists.oasis-open.org
Date: Tue, 4 Oct 2005 11:10:03 -0400

Hello

this is the element definition in the sequence inside polisy and policyset elements:
<xs:element ref="xacml:VariableDefinition"/>

why isn't minOccurs equal to zero? it looks to me like if it had to be optional.

thanks,
argyn