Double Negation/Double Negation Introduction/Proof Rule

Proof Rule
The rule of double negation introduction is a valid deduction sequent in propositional logic.

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

It can be written:
 * $\displaystyle {\phi \over \neg \neg \phi} \neg \neg_i$