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unless: except if (used to introduce a case in which a statement being made is not true or valid).
refute: prove (a statement or theory) to be wrong or false; disprove.
assert: state a fact or belief confidently and forcefully.
if: introducing a conditional clause.
上述解释引自 New Oxford American Dictionary
简单的条件判断时 unless 感脚不错,复杂的组合条件判断时,unless 完全就是一深埋坑。
我很喜欢 unless,翻译成 如果不 和 如果没有