This is INVALID.

See how two snakes have been added to part of a formula. This is not allowed by the rule.

The correct application of the rule adds or removes two adjacent negation signs ("tildes," "snakes," whatever.) to a complete formula. There is no rule that allows alterations to just part of a formula.

Before you go back to the reading, consider the following two attempts to apply NN. Are they both good? Both bad? Or what?

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