Euclidean algorithm (calculating the gcd)

 a 20405 ← b 84645 ← x a abs ← y b abs ← x 0 ≠ ( y 0 ≠ ( & x y > x x y mod ← y y x mod ← if while GCD x y + ← 2 1 line GCD 55 =

Control:

 a GCD / 371 = b GCD / 1539 =