def undo(self) -> None:
"""Create a new item and recover expression."""
item = QListWidgetItem(self.name)
item.expr = self.mechanism
item.setIcon(QIcon(QPixmap("icons:mechanism.png")))
self.widget.insertItem(self.row, item)
def redo(self) -> None:
"""Add mechanism expression to 'expr' attribute."""
item = QListWidgetItem(self.name)
item.expr = self.mechanism
item.setIcon(QIcon(QPixmap("icons:mechanism.png")))
self.widget.addItem(item)
