FUNCTION fibonacci (n)
    LET n1 = 0
    LET n2 = 1
    FOR k = 1 TO ABS(n)
        LET sum = n1 + n2
        LET n1 = n2
        LET n2 = sum
    NEXT k
    IF n < 0 THEN
       LET fibonacci = n1 * ((-1) ^ ((-n) + 1))
    ELSE
       LET fibonacci = n1
    END IF
END FUNCTION

PRINT fibonacci(0)      ! 0
PRINT fibonacci(13)     ! 233
PRINT fibonacci(-42)    !-267914296
PRINT fibonacci(47)     ! 2971215073
END