Greatest common divisor of two integers.
| a | Number 1 for the algorithm. | 
| b | Number 2 for the algorithm. | 
| return value | The greatest common divisor of a and b. | 
It's based on Euclid's algorithm.
The arguments should be integers.
The algorithm can have more arguments, and the greatest common divisor of all specified numbers will be returned.
	gcd(16, 8*3)