{{alias}}( W )
    Returns an accumulator function which incrementally computes a moving mid-
    range.

    The mid-range is the arithmetic mean of maximum and minimum values.
    Accordingly, the mid-range is the midpoint of the range and a measure of
    central tendency.

    The `W` parameter defines the number of values over which to compute
    the moving mid-range.

    If provided a value, the accumulator function returns an updated moving mid-
    range. If not provided a value, the accumulator function returns the current
    moving mid-range.

    As `W` values are needed to fill the window buffer, the first `W-1` returned
    values are calculated from smaller sample sizes. Until the window is full,
    each returned value is calculated from all provided values.

    Parameters
    ----------
    W: integer
        Window size.

    Returns
    -------
    acc: Function
        Accumulator function.

    Examples
    --------
    > var accumulator = {{alias}}( 3 );
    > var mr = accumulator()
    null
    > mr = accumulator( 2.0 )
    2.0
    > mr = accumulator( -5.0 )
    -1.5
    > mr = accumulator( 3.0 )
    -1.0
    > mr = accumulator( 5.0 )
    0.0
    > mr = accumulator()
    0.0

    See Also
    --------