This is INVALID.
See how only one snake
has been removed. This is wrong.
The correct application of the rule adds or removes two adjacent tildes ("snakes," "negation signs," whatever) to a complete formula. Messing with just one negation sign 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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