class Integer

def prime?
    num = self.abs
    return false if num == 0 || num == 1
    return true if (1..num-1).inject {|acc, val| (acc * val) % num} == num - 1
    false
end

end

#Uses Wilson's theorem to check for primality.
#Pros: Is always correct
#Cons: Even less efficient than trial division - not practical at all