# Idempotency of entailment

**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,

In linear and affine logic, entailment is not idempotent.

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