{{alias}}( x )
    Computes the inverse half-value versed cosine.

    The inverse half-value versed cosine is defined as `2*acos(sqrt(x))`.

    If `x < 0`, `x > 1`, or `x` is `NaN`, the function returns `NaN`.

    Parameters
    ----------
    x: number
        Input value.

    Returns
    -------
    y: number
        Inverse half-value versed cosine.

    Examples
    --------
    > var y = {{alias}}( 0.5 )
    ~1.5708
    > y = {{alias}}( 0.0 )
    ~3.1416

    See Also
    --------