{{alias}}( σ )
    Returns the standard deviation of a half-normal distribution.

    If provided `σ <= 0`, the function returns `NaN`.

    Parameters
    ----------
    σ: number
        Scale parameter.

    Returns
    -------
    out: number
        Standard deviation.

    Examples
    --------
    > var v = {{alias}}( 9.0 )
    ~5.425
    > v = {{alias}}( 2.0 )
    ~1.2056

    See Also
    --------
