A Mersenne prime is a prime number of the form M = 2^p - 1, where p is another prime number. For example, 31 is a Mersenne prime because 31 = 2^5 - 1 and both 31 and 5 are prime numbers.

Implement the function isMersenne(x) so that it returns true if x is a Mersenne prime and false otherwise. Your solution should work for all positive integer values of x less than 1,000,000,000 (one billion).