Algorithm description
---------------------
Various parallel GCD algorithms exist. In this challenge, we consider a
simple Euclid-like algorithm with two parallel threads. One thread
subtracts in one direction, the other thread subtracts in the other
direction, and eventually this procedure converges on GCD.
Implementation
--------------
In pseudocode, the algorithm is described as follows:
(
WHILE a != b DO
IF a>b THEN a:=a-b ELSE SKIP FI
OD
||
WHILE a != b DO
IF b>a THEN b:=b-a ELSE SKIP FI
OD
);
OUTPUT a
