def backtrace
      return nil if super.nil?
      return super if sass_backtrace.all? {|h| h.empty?}
      sass_backtrace.map do |h|
        "#{h[:filename] || '(sass)'}:#{h[:line]}" +
          (h[:mixin] ? ":in `#{h[:mixin]}'" : "")
      end + super
    end
def backtrace
      return nil if super.nil?
      return super if sass_backtrace.all? {|h| h.empty?}
      sass_backtrace.map do |h|
        "#{h[:filename] || '(sass)'}:#{h[:line]}" +
          (h[:mixin] ? ":in `#{h[:mixin]}'" : "")
      end + super
    end
Returns the standard exception backtrace,
 including the Sass backtrace.

 @return [Array<String>]