docs: private OrderedHashSet getSimpleColumnNames(java.util.List<Expression> indexExprs) {
          OrderedHashSet set = new OrderedHashSet();
  
          for (Expression expression : indexExprs) {
              if (expression instanceof ExpressionColumn) {
                  String colName = ((ExpressionColumn)expression).columnName;
                  if (!set.add(colName)) {
                      throw Error.error(ErrorCode.X_42577, colName);
                  }
              } else {
                  return null;
              }
          }
  
          return set;
      }