*/
public static Set<Column> getColumnsReferencedBy( Visitable visitable ) {
if (visitable == null) return Collections.emptySet();
final Set<Column> symbols = new HashSet<Column>();
// Walk the entire structure, so only supply a StrategyVisitor (that does no navigation) ...
Visitors.visitAll(visitable, new AbstractVisitor() {
protected void addColumnFor( SelectorName selectorName,
String property ) {
symbols.add(new Column(selectorName, property, property));
}