受约人不能既有承诺的表演，也不能有他同意接受的其他表演。
The covenantee cannot have both the promised performance and some other performance which he agrees to accept.
对价一般由受约人提供，但不一定提供给立约人；
Consideration is provided by the promisee, though not necessarily to the promisor;
对价一般由受约人提供，但不一定提供给立约人；
Consideration is provided by the promisee, though not necessarily to the promisor;
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