The PhantomReference class is useful only to track the impending collection of the referring object.

phantomreference类只能用于跟踪对被引用对象即将进行的收集。

youdao

And what I referring to wasn't just the caboose but the whole long extended-through-space object.

我指的不仅仅是车尾,而是整个长长的,贯穿空间的物体。

耶鲁公开课 - 死亡课程节选

The fourth "d" here is time, referring to an object that, once printed, is capable of changing shape (over time, naturally).

ENGADGET: 3D? Feh. MIT has already moved on to 4D printing (video) Alt