def step(pos_x, pos_y, vx, vy):
    """
    The probe's x,y position starts at 0,0. Then, it will follow some trajectory by moving in steps. 

    On each step, these changes occur in the following order:

    - The probe's x position increases by its x velocity.
    - The probe's y position increases by its y velocity.
    - Due to drag, the probe's x velocity changes by 1 toward the value 0; that is, 
        - it decreases by 1 if it is greater than 0, 
        - increases by 1 if it is less than 0, 
        - or does not change if it is already 0.
    - Due to gravity, the probe's y velocity decreases by 1.
    """
    pos_x += vx
    pos_y += vy
    if vx > 0:
        vx -= 1
    elif vx < 0:
        vx += 1
    vy -= 1

    return pos_x, pos_y, vx, vy