code: function luminosity(color){
    utils.assertColor(color);
    color = color.rgba;
    function processChannel(channel) {
      channel = channel / 255;
      return (0.03928 > channel)
        ? channel / 12.92
        : Math.pow(((channel + 0.055) / 1.055), 2.4);
    }
    return new nodes.Unit(
      0.2126 * processChannel(color.r)
      + 0.7152 * processChannel(color.g)
      + 0.0722 * processChannel(color.b)
    );
  }
docs: Returns the relative luminance of the given `color`,
  see http://www.w3.org/TR/WCAG20/#relativeluminancedef
  
  Examples:
  
  luminosity(white)
  // => 1
  
  luminosity(#000)
  // => 0
  
  luminosity(red)
  // => 0.2126
  
  @param {RGBA|HSLA} color
  @return {Unit}
  @api public