In your implementation, the parameters are of type
`java.math.BigInteger`.
Both mathematical methods that I am asking you to implement
are in the Java library, so you can use the Java
library to check your answers.
However, you should only be using the following `BigInteger`
methods: `mod`, `add`, `equals`, `compareTo`,
`multiply`, `shiftRight`.

- Greatest Common Divisior: the greatest common divisor of
`m`and`n`, written`gcd(m,n)`is the largest integer that divides both. We assume that both`m`and`n`are positive. If either`m`or`n`is 1, then the greatest common divisor is 1. For instance,gcd(1,10) = 1 gcd(5,15) = 5 gcd(10,10) = 10 gcd(10,15) = 5 gcd(24,50) = 2 gcd(100,81) = 1

Note that if`m`is larger than`n`,`gcd(m,n)=gcd(n,m%n)`. - Power: Raise a number to a non-negative power.
To compute
`pow(x,n)`note that if`n`is even (but not zero), then you can compute`pow(x*x,n/2)`, and if`n`is odd, then you can compute`pow(x,n-1)*x`.