public void addColumnToModify(Column column){
  if (!columnsToModify.contains(column)) {
    columnsToModify.add(column);
  }
}

public void addColumnToModify ( Column column ) { if ( ! columnsToModify . contains ( column ) ) { columnsToModify . add ( column ) ; } }
adds a column that has been modified between the two dbs .
