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.
A gel manicure and a Shellac manicure are essentially the same thing: long-lasting polishes cured under UV lamps. The difference is that gel nails or manicure is the generic term for nails done under a UV lamp, while Shellac is a brand of gel nails by the company Creative Nail Design (CND).