def seiffert_mean(nums):
if len(nums) == <NUM_LIT:1>:<EOL><INDENT>return nums[<NUM_LIT:0>]<EOL><DEDENT>if len(nums) > <NUM_LIT:2>:<EOL><INDENT>raise AttributeError('<STR_LIT>')<EOL><DEDENT>if nums[<NUM_LIT:0>] + nums[<NUM_LIT:1>] == <NUM_LIT:0> or nums[<NUM_LIT:0>] - nums[<NUM_LIT:1>] == <NUM_LIT:0>:<EOL><INDENT>return float('<STR_LIT>')<EOL><DEDENT>return (nums[<NUM_LIT:0>] - nums[<NUM_LIT:1>]) / (<EOL><NUM_LIT:2> * math.asin((nums[<NUM_LIT:0>] - nums[<NUM_LIT:1>]) / (nums[<NUM_LIT:0>] + nums[<NUM_LIT:1>]))<EOL>)<EOL>
r"""Return Seiffert's mean.

    Seiffert's mean of two numbers x and y is:
    :math:`\frac{x - y}{4 \cdot arctan \sqrt{\frac{x}{y}} - \pi}`

    It is defined in :cite:`Seiffert:1993`.

    Parameters
    ----------
    nums : list
        A series of numbers

    Returns
    -------
    float
        Sieffert's mean of nums

    Raises
    ------
    AttributeError
        seiffert_mean supports no more than two values

    Examples
    --------
    >>> seiffert_mean([1, 2])
    1.4712939827611637
    >>> seiffert_mean([1, 0])
    0.3183098861837907
    >>> seiffert_mean([2, 4])
    2.9425879655223275
    >>> seiffert_mean([2, 1000])
    336.84053300118825