public static function darken($color, $amount){
if( $color instanceof Less_Tree_Color ){
$hsl = $color->toHSL();
$hsl['l'] -= $amount->value / 100;
$hsl['l'] = self::clamp($hsl['l']);
return Less_Environment::hsla($hsl['h'], $hsl['s'], $hsl['l'], $hsl['a']);
}
Less_Environment::Expected('color',$color);
}