def grayscale(color)
      if color.is_a?(Sass::Script::Value::Number)
        return identifier("grayscale(#{color})")
      end
      desaturate color, number(100)
    end
def grayscale(color)
      if color.is_a?(Sass::Script::Value::Number)
        return identifier("grayscale(#{color})")
      end
      desaturate color, number(100)
    end
Converts a color to grayscale. This is identical to `desaturate(color,
 100%)`.

 @see #desaturate
 @overload grayscale($color)
   @param $color [Sass::Script::Value::Color]
 @return [Sass::Script::Value::Color]
 @raise [ArgumentError] if `$color` isn't a color