code: function hsla(hue, saturation, lightness, alpha){
    switch (arguments.length) {
      case 1:
        utils.assertColor(hue);
        return hue.hsla;
      case 2:
        utils.assertColor(hue);
        var color = hue.hsla;
        utils.assertType(saturation, 'unit', 'alpha');
        var alpha = saturation.clone();
        if ('%' == alpha.type) alpha.val /= 100;
        return new nodes.HSLA(
            color.h
          , color.s
          , color.l
          , alpha.val);
      default:
        utils.assertType(hue, 'unit', 'hue');
        utils.assertType(saturation, 'unit', 'saturation');
        utils.assertType(lightness, 'unit', 'lightness');
        utils.assertType(alpha, 'unit', 'alpha');
        var alpha = alpha.clone();
        if (alpha && '%' == alpha.type) alpha.val /= 100;
        return new nodes.HSLA(
            hue.val
          , saturation.val
          , lightness.val
          , alpha.val);
    }
  }
docs: Convert the given `color` to an `HSLA` node,
  or h,s,l,a component values.
  
  Examples:
  
  hsla(10deg, 50%, 30%, 0.5)
  // => HSLA
  
  hsla(#ffcc00)
  // => HSLA
  
  @param {RGBA|HSLA|Unit} hue
  @param {Unit} saturation
  @param {Unit} lightness
  @param {Unit} alpha
  @return {HSLA}
  @api public