Alternative Endings Not Appearing

[S L Raymond]

I'm using Lilypond 2.6, and alternative ending brackets are not showing
up.

The rest of the score looks just fine.  I double-checked my syntax
against the manual, and it seems I should be doing it right.  Ideas?

Here's the score:

\header {
        title = "Isthmus"
        composer = "S L Raymond"
        pagenumber = "no"
        tagline = " "
        }

changes = \chords {r1 c:7 r r r bes:7 r r a:9 g:7 f:7 es:7 des:7 g:7 f:7
es:7 des:7}

\score {
   <<
     \context ChordNames \with {voltaOnThisStaff = ##t } {
         \transpose c d \changes
     }
     \context Staff = one \with { voltaOnThisStaff = ##f} \relative
d'' {
         \set Staff.instrument = #"Tenor Sax"
        \key d \major
        \time 4/4
        \clef treble
        \skip 2
                a8 c e d
                \repeat volta 2 {
                        r1
                        r2 r4 b'4
                        c8 dis,8 e g b a r8 fis
                        r8 g,4. bes4 d8 c~
                        c1
                        r2 r8 e8 g a
                        bes a g f e d r8 g~
                        g4 r4 d8 c b a
                }
                \alternative {
                        { r8
                        a b c cis a' aes g~
                        g2 b,8 d g f~
                        f1~
                        f2 a,8 c e d
                         } 
                        { r8
                        a b c cis a' aes g~
                        g2 b,8 d g f~
                        f1~
                        f \bar".|"}
                        }
                }

        \new Staff \relative d' {
                \set Staff.instrument = #"Guitar"
                \key c \major
                \clef treble
                \skip 2
                g8 bes d c
                \repeat volta 2 {
                r8 <es bes e,>4.^\accent~ <es bes e,>2~ 
                <es bes e,>2 r4 a
                bes8 cis, d f a g r e 
                r8 f,4. aes4 c8 bes~
                bes4 r8 <des aes d,>^\accent~ <des aes d,>2~
                <des aes d,> r8 d f g aes g f es d c r f~
                f4 r c8 bes a g }
                \alternative { {
                 r8 g a ais b g' ges f~ 
                f2 a,8 c f <es des g,>~
                <es des g,>1  
                 \override Voice.NoteHead #'style = #'slash
                  \override Voice.Stem #'transparent = ##t
                b4 b    
                  \override Voice.Stem #'transparent = ##f
                \override Voice.NoteHead #'style = #'default
                g8 bes d c}
                {
                r8 g a ais b g'_\markup {\small {improvise harmony}}  ges f~ 
                f2 a,8 c f <es des g,>~
                <es des g,>1~ <es des g,> } }
        }

     \new ChordNames {
         \changes
     }
     \new Staff \relative d {
                \set Staff.instrument = #"Bass"
                \key c \major
                \clef bass
                \skip 2
                r2
                \repeat volta 2 {
                        r8 c8^\accent~ c4 ~ c2~
                        c1~
                        c1~
                        c2 r2

                        r4. bes8^\accent~ bes2~
                        bes1~
                        bes1
                        a2
                 \override Voice.NoteHead #'style = #'slash
                  \override Voice.Stem #'transparent = ##t
                        d4 d }
                \alternative { {
                        d d d d
                        d d d d
                        d d d d
                        d d d d } {
                        d_\markup "Last Time: repeat final 4 bars"
                        d
                        d d 
                        d d d d 
                        d d d d
                        d d d d
                        } }
        }
 >>
}