Why doesn't keysDownRepeat() in libnds seem to work when calling it multiple times?

I have code like this to move the player in my game left, right, up, and down:

keysSetRepeat(20, 5);

while (lives) {
    scanKeys();

    if (keysDownRepeat() & (KEY_LEFT | KEY_RIGHT | KEY_UP | KEY_DOWN)) {
        u8 new_x = x;
        u8 new_y = y;

        if (keysDownRepeat() & KEY_LEFT) {
            new_x--;
        } else if (keysDownRepeat() & KEY_RIGHT) {
            new_x++;
        } else if (keysDownRepeat() & KEY_DOWN) {
            new_y++;
        } else if (keysDownRepeat() & KEY_UP) {
            new_y--;
        }

        // ...
    }

    // ...

    swiWaitForVBlank();
}

Why are the keys not being detected? If I replace keysDownRepeat() with keysDown() it works (without the repeat rate, of course). The documentation is no help here.

I had to find the libnds source code to figure this out. Look at the implementation of keysDownRepeat():

uint32 keysDownRepeat(void) {
    uint32 tmp = keysrepeat;

    keysrepeat = 0;

    return tmp;
}

It actually returns the keys then resets them back to 0. This wasn't documented. I solved this by storing the result of keysDownRepeat() into a variable and using the variable to check the keys:

keysSetRepeat(20, 5);

while (lives) {
    scanKeys();
    u32 keys_down_repeat = keysDownRepeat();

    if (keys_down_repeat & (KEY_LEFT | KEY_RIGHT | KEY_UP | KEY_DOWN)) {
        u8 new_x = x;
        u8 new_y = y;

        if (keys_down_repeat & KEY_LEFT) {
            new_x--;
        } else if (keys_down_repeat & KEY_RIGHT) {
            new_x++;
        } else if (keys_down_repeat & KEY_DOWN) {
            new_y++;
        } else if (keys_down_repeat & KEY_UP) {
            new_y--;
        }

        // ...
    }

    // ...

    swiWaitForVBlank();
}