{{alias}}( x, y[, scale] )
    Computes the relative difference of two real numbers in units of double-
    precision floating-point epsilon.

    By default, the function scales the absolute difference by dividing the
    absolute difference by the maximum absolute value of `x` and `y`. To scale
    by a different function, specify a scale function name.

    The following `scale` functions are supported:

    - 'max-abs': maximum absolute value of `x` and `y` (default).
    - 'max': maximum value of `x` and `y`.
    - 'min-abs': minimum absolute value of `x` and `y`.
    - 'min': minimum value of `x` and `y`.
    - 'mean-abs': arithmetic mean of the absolute values of `x` and `y`.
    - 'mean': arithmetic mean of `x` and `y`.
    - 'x': `x` (*noncommutative*).
    - 'y': `y` (*noncommutative*).

    To use a custom scale function, provide a function which accepts two numeric
    arguments `x` and `y`.

    If computing the relative difference in units of epsilon will result in
    overflow, the function returns the maximum double-precision floating-point
    number.

    If the absolute difference of `x` and `y` is `0`, the relative difference is
    always `0`.

    If `|x| = |y| = infinity`, the function returns `NaN`.

    If `|x| = |-y| = infinity`, the relative difference is `+infinity`.

    If a `scale` function returns `0`, the function returns `NaN`.

    Parameters
    ----------
    x: number
        First number.

    y: number
        Second number.

    scale: string|Function (optional)
        Scale function. Default: `'max-abs'`.

    Returns
    -------
    out: number
        Relative difference in units of double-precision floating-point epsilon.

    Examples
    --------
    > var d = {{alias}}( 12.15, 12.149999999999999 )
    ~0.658
    > d = {{alias}}( 2.4341309458983933, 2.4341309458633909, 'mean-abs' )
    ~64761.512

    // Custom scale function:
    > function scale( x, y ) { return ( x > y ) ? y : x; };
    > d = {{alias}}( 1.0000000000000002, 1.0000000000000100, scale )
    ~44

    See Also
    --------