Redefining \parenthesisOpenSymbol

[Vaughan McAlley]

Most scores I do (renaissance choral editions) have no slurs and
hundreds of \melisma and \melismaEnds. My source code would look a lot
nicer if I could redefine ( and ) to mean \melisma and \melismaEnd.
What is wrong with this code?

\version "2.16.1"

slurStart = #(make-span-event 'SlurEvent START)
slurEnd = #(make-span-event 'SlurEvent STOP)
parenthesisOpenSymbol = \melisma
parenthesisCloseSymbol = \melismaEnd

\score {
  <<
    \new Staff {
      \new Voice = "musicNotes" {
        \relative c' {
          c8 \slurStart d e f \slurEnd % slurred
          g ( a b c ) % no slur, but with melisma
          c,1
        }
      }
    }
    \new Lyrics \lyricsto "musicNotes" {
      Li -- ly -- pond.
    }
  >>
}

(By the way, removing the slur engraver is no help—the pages with
their long melismas are vertically spaced as if the huge slurs were
there)

Thanks,
Vaughan