一元操作符负号和二元操作符减号的区别是什么？
What is the difference between the unary minus operator and the binary subtraction operator?
在Scala中定义这种一元“操作符”需要一点技巧，但这只是语法上的问题而已。
Defining this unary "operator" in Scala is slightly tricky, but it's purely a syntactic nit.
在我们的四则运算表达式里面我们可以利用继承实行从一元到二元操作符的表示，但它实质上会干扰测试。
In our arithmetic expressions we could have used inheritance to get from a unary to a binary operator node, but that would substantially violate the test.
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