Implement `String#digit?` (in Java `StringUtils.isDigit(String)`), which should return `true` if given object is a digit (0-9), `false` otherwise.
def is_digit(n):
    #your code here: shortest variation
    return n in set('0123456789')
def is_digit(n):
    #your code here: shortest variation
    return n in set('0123456789')