big x^y modulo function

provide the solution when the normal mod function can't because the input is to big

In some fields as cryptology (RAS-cryption), there is a need of doing x^y mod m where x, y and n are huge. The build-in maltab function can't achieve the result because it first calculate x^y and found it is equals to inf. The hereunder function provides a performent solution to this problem and gives the right answer...