Re: [xacml] concrete proposal of condition reference (#7)

A <Function> element should have exactly the same schema definition as
<VariableReference>. A <VariableReference> is an element that extends
ExpressionType and has a single required XML attribute called
"VariableId".  A <Function> element should have an analogous definition,
i.e. it should be an element that extends ExpressionType and has a single
required XML attribute called "FunctionId".

Cheers,
-Polar