An inference of natural deduction is a normal form, according to Dag Prawitz, if no formula occurrence is both the principal premise of an elimination rule and the conclusion of an introduction rule.[1]

References

  1. ↑ Prawitz, Dag (2006-02-24). Natural Deduction: A Proof-Theoretical Study. Courier Dover Publications. ISBN 978-0-486-44655-4.


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