اوپال (زبان برنامه‌نویسی)

    SIGNATURE GCD
    FUN GCD: nat ** nat -> nat

   IMPLEMENTATION GCD
   IMPORT Nat COMPLETELY
   DEF GCD(a,b) == IF a % b = 0 THEN b
                       ELSE IF a-b <b THEN GCD(b,a-b)
                           ELSE GCD(a-b,b)
                       FI
                   FI