This is VALID.
Notice how application of the rule
adds two adjacent "snakes" (okay, they're called "tildes," but I like snakes.)
The correct application of the rule adds or removes two adjacent tildes ("snakes," "negation signs," whatever) to a complete formula.
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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