code: function lightness(color, value){
    if (value) {
      var hslaColor = color.hsla;
      return hsla(
        new nodes.Unit(hslaColor.h),
        new nodes.Unit(hslaColor.s),
        value,
        new nodes.Unit(hslaColor.a)
      )
    }
    return component(color, new nodes.String('lightness'));
  }
docs: Return the lightness component of the given `color`,
  or set the lightness component to the optional second `value` argument.
  
  Examples:
  
  lightness(#00c)
  // => 100%
  
  lightness(#00c, 80%)
  // => #99f
  
  @param {RGBA|HSLA} color
  @param {Unit} [value]
  @return {Unit|RGBA}
  @api public