Double Negation/Double Negation Elimination/Proof Rule

Theorem
The rule of Double Negation Elimination is a valid deduction sequent in propositional logic.

As a proof rule it is expressed in the form:
 * If we can conclude $\neg \neg \phi$, then we may infer $\phi$.

It can be written:
 * $\ds {\neg \neg \phi \over \phi} \neg \neg_e$