Having the prototype available before the first use of the function allows the compiler to check that the correct number and type of arguments are used in the function call and that the returned value, if any, is being used reasonably. The function definition itself can act as an implicit function declaration.
While shellac is not usually the finish paint choice, it is often thought of as the best paint primer available. It has properties that oil and water based paints can't match, and provides a more dependable seal than most other primers are able to provide.
I own a salon. Shellac does not wreck your nails unless you remove it improperly. The process takes about 30 minutes if you can get right in. Also, if the salon has LED lights it takes a fraction of the time to cure than UV. Also, if it's a color that requires multiple coats, could take longer as each coat has to cure.