" They'd never have set a task that was undoable."
“他们不可能设计一个无法完成的项目。”
英语百科
Undo
（重定向自Undoable）
Undo is a command in many computer programs. It erases the last change done to the document reverting it to an older state. In some more advanced programs such as graphic processing, undo will negate the last command done to the file being edited.
The opposite of undo is redo. The redo command reverses the undo or advances the buffer to a more current state.