Can't get \arpeggio to work

[ivan . k . kuznetsov]

According to the reference manual,
http://lilypond.org/doc/v2.13/Documentation/notation/expressive-marks-as-lines#arpeggio

all I need to do is append the \arpeggio tag to a chord construct,
as I do in my appended snippet.  However, the following snippet
produces the message:

   issue02a.ly:11:41: error: syntax error, unexpected EVENT_IDENTIFIER
     << bf,4 gs4 b4 >>  << a,4 bf4 d'4 >>
                                         \arpeggio
   issue02a.ly:4:0: error: errors found, ignoring music expression


Can someone advise?  Thanks!


\version "2.12.3"
\include "english.ly"

<<    
   \new Staff = "guitar"
   {
     \clef "treble_8"
     \time 2/4

     << bf,4 gs4 b4 >>  << a,4 bf4 d'4 >>\arpeggio
   } 
>>