def data(self, index, role = QtCore.Qt.DisplayRole):
        if (not index.isValid()
            and index.row() >= len(self._order.rows)
            and index.column() >= 0 and index.column < self.columnCount()
            and role == QtCore.Qt.DisplayRole):
            return QVariant("%d/%d"%(index.column(), index.row()))
        return QVariant("error")


    def headerData(self, section, orientation, role = QtCore.Qt.DisplayRole):
        if role == QtCore.Qt.DisplayRole:            
            if orientation == QtCore.Qt.Horizontal:
                return QtCore.QVariant(self._columnTitles[section])
            else:
                return QtCore.QVariant("Row %d"%section)
        return QtCore.QVariant()