# Idempotency of entailment

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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. This property can be captured by a structural rule called **contraction**, and in such systems one may say that entailment is idempotent if and only if contraction is an admissible rule.

Rule of Contraction: from

- A,C,C → B

is derived

- A,C → B.

Or in sequent calculus notation,

## See also[edit]

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