code: public void addColumn(SchemaColumn column) {
          int size = m_columns.size();
          m_columnsMapHelper.put(column, size);
          m_columns.add(column);
      }
docs: Add a column to this schema.
  
  Unless actively modified, updated, or sorted, the column order is
  implicitly the order in which columns are added using this call.
  
  Note that it's possible to add the same column to a schema more than once.
  In this case we replace the old entry for the column in the map (so it will
  stay the same size), the column list will grow by one, and the updated map entry
  will point the the second instance of the column in the list.