{{alias}}( α )
    Returns an accumulator function which incrementally computes an
    exponentially weighted mean, where α is a smoothing factor between 0 and 1.

    If provided a value, the accumulator function returns an updated mean. If
    not provided a value, the accumulator function returns the current mean.

    If provided `NaN` or a value which, when used in computations, results in
    `NaN`, the accumulated value is `NaN` for all future invocations.

    Parameters
    ----------
    α: number
        Smoothing factor (value between 0 and 1).

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

    Examples
    --------
    > var accumulator = {{alias}}( 0.5 );
    > var v = accumulator()
    null
    > v = accumulator( 2.0 )
    2.0
    > v = accumulator( -5.0 )
    -1.5
    > v = accumulator()
    -1.5

    See Also
    --------