/**
* Decreases a number by one
* @param x number that should be decreased
* @return   number decreased by one
*/
public int decrease(int x) {...}

/**
* Calculates the factorial of a number. The number must be between 0 and 20,
* else it won't calculate.
* @param n number that should be calculated
* @return   factorial of the number
*/
public int factorial(int n) {...}