This is INVALID.
See how only one tilde has been added. This is wrong.
The correct application of the rule adds or removes two adjacent negation signs ("tildes," "snakes," whatever.) to a complete formula. Messing with just one snake is not allowed by the rule.
Before you go back to the reading, consider the following two attempts to apply NN. Are they both good? Both bad? Or what?
Use your browsers "back" key to return.
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