def opacity(color)
      if color.is_a?(Sass::Script::Value::Number)
        return identifier("opacity(#{color})")
      end
      assert_type color, :Color, :color
      number(color.alpha)
    end
def opacity(color)
      if color.is_a?(Sass::Script::Value::Number)
        return identifier("opacity(#{color})")
      end
      assert_type color, :Color, :color
      number(color.alpha)
    end
Returns the alpha component (opacity) of a color. This is 1 unless
 otherwise specified.

 @overload opacity($color)
   @param $color [Sass::Script::Value::Color]
 @return [Sass::Script::Value::Number] The alpha component, between 0 and 1
 @raise [ArgumentError] if `$color` isn't a color