Making LilyPond accidentals consume no horizontal space

Accidentals consume no horizontal space with their X-extent set to zero.

   \new Staff {
      \set Score.proportionalNotationDuration = #(ly:make-moment 1 32)
      \override Accidental #'X-extent = #'(0 . 0)
      \override Accidental #'extra-offset = #'(-1 . 0)
      c'16
      cis'16
      c'16
      c'16
      c'16
      c'16
      c'16
      c'16
   }

extra-offset is required to move zero-width accidentals away from the left edge of the parent notehead.