# File lib/sass/stack.rb, line 101
    def to_s
      Sass::Util.enum_with_index(Sass::Util.enum_cons(frames.reverse + [nil], 2)).
          map do |(frame, caller), i|
        "#{i == 0 ? 'on' : 'from'} line #{frame.line}" +
          " of #{frame.filename || 'an unknown file'}" +
          (caller && caller.name ? ", in `#{caller.name}'" : "")
      end.join("\n")
    end