def pull_top(piles, pos):
    """
    Pulls the top card of pile number `pos`,
    and returns the new set of piles.

    Example:
    > pull_top([[1,2,3], [], [4,5,6], [7,8,9]], 2)
    [[1,2,3], [], [4,5], [7,8,9]]
    """
    rest = piles[:]
    rest[pos] = piles[pos][:-1]
    return rest