合同当事人互负债务,先后履行顺序已有约定,后履行义务的当事人可在先履行义务的当事人未履行之前或履行有暇疵时,拒绝对方的履行请求,合同法称此为( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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