docs: private OrderedHashSet getBaseColumnNames(java.util.List<Expression> indexExprs) {
          OrderedHashSet set = new OrderedHashSet();
  
          HsqlList col_list = new HsqlArrayList();
          for (Expression expression : indexExprs) {
              expression.collectAllColumnExpressions(col_list);
          }
  
          for (int i = 0; i < col_list.size(); i++) {
              String colName = ((ExpressionColumn)col_list.get(i)).columnName;
              set.add(colName);
          }
  
          return set;
      }