returns the GCD (greatest common divisor) of A and B, which must
be one of the integer data types. A and B may be conformable
arrays; the semantics of the gcd call are the same as any other
binary operation. Uses Euclid's celebrated algorithm.
The absolute values of A and B are taken before the operation
commences; if either A or B is 0, the return value will be 0.
interpreted function, defined at i/gcd.i line 10