final Set items = new java.util.HashSet();
boolean more = false;
ChoiceExp ch = (ChoiceExp)constraint;
Expression[] children = ch.getChildren();
for( int i=0; i<children.length; i++ ) {
if( children[i] instanceof ValueExp )
items.add( ((ValueExp)children[i]).value.toString() );
else
// this is a fairly complex expression