code: private void addColumn(String storeName, String rowKey, String colName, byte[] colValue) {
          Map<String, Map<String, byte[]>> rowMap = m_columnAdds.get(storeName);
          if (rowMap == null) {
              rowMap = new HashMap<>();
              m_columnAdds.put(storeName, rowMap);
          }
          
          Map<String, byte[]> colMap = rowMap.get(rowKey);
          if (colMap == null) {
              colMap = new HashMap<>();
              rowMap.put(rowKey, colMap);
          }
          
          byte[] oldValue = colMap.put(colName, colValue);
          if (oldValue == null) {
              m_totalUpdates++;
          } else if (!Arrays.equals(oldValue, colValue)) {
              m_logger.debug("Warning: duplicate column mutation with different value: " +
                             "store={}, row={}, col={}, old={}, new={}",
                             new Object[]{storeName, rowKey, colName, oldValue, colValue});
          }
      }
docs: Add the given column update; the value may be null.