I mean, again, 99% of the vector fields that we see in this class are defined everywhere so that's not an urgent concern, but still.
再次强调在,课堂上我们看到的99%的向量场,都是处处有定义的,就不必很担心了,但是还得注意。
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Data: An object; a generic reference that is carried along to listeners; often a collection class (such as a Vector or Hashtable) or a customized data object.