unary expression 一元表达式
Unary operators perform an operation on a single operand, such as returning the negative or positive value of a numeric expression.
一元运算符对单个操作数执行操作，如返回数值表达式的负值或正值。
Performs a unary operation that returns the negative value of a numeric expression.
执行一元运算，返回数值表达式的负值。
Note that numeric literals do not include a sign; a phrase like -1 is actually an expression composed of the unary operator '-' and the literal 1.
注意数值型的字面值不包括符号(译注：正负号)，像- 1实际上是个组合了一元运算符“-”和字面值1的表达式。
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