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Idempotency of entailment is a property of logical systems that states that one may derive the same consequences from many instances of a hypothesis as from just one. In sequent calculi this property can be captured by a structural rule called contraction and in such systems one may say that entailment is idempotent just in case contraction is an admissible rule. Rule of Contraction: from A,C,C -> B is derived
A,C -> B. Or in sequent calculus notation,
- <math>\frac{\Gamma,C,C\vdash B}{\Gamma,C\vdash B}</math>
