/* n = r0, k = r1, n >=k >=0 */
binom:
    /* if (k == 0 || n == k) then 1 */
    cmp r1, #0
    cpmne r0, r1
    addeq r2, r2, #1
    bxeq lr

    /* left recursive call */
    sub r0, r0, #1
    push {r0,r1,lr}
    bl binom

    /* right recursive call */
    pop {r0,r1}
    sub r1, r1, #1
    bl binom

    /* cumulative result */
    pop {lr}
    bx lr

main:
    ldr r0, =5
    ldr r1, =3
    ldr r2, =0
    push {lr}
    bl binom
    mov r0, r2
    pop {lr}
    bx lr