\version "2.17.11"

#(define (conditional-kill-lyric-extender-callback . args)
  (lambda (grob)
   (let* ((orig (ly:grob-original grob))
	  (siblings (ly:spanner-broken-into orig))
          (minimum-length
           (if (null? args)
            (ly:grob-property grob 'minimum-length 0)
            (car args)))
          (X-extent (ly:stencil-extent (ly:grob-property grob 'stencil empty-stencil) X))
          (natural-length (- (cdr X-extent) (car X-extent))))
    (if (and (> minimum-length natural-length)
	 (<= (length siblings) 1)) ;; never kill a broken extender
     (ly:grob-suicide! grob)))))

\layout {
  \context {
    \Lyrics
    \override LyricExtender.minimum-length = #1.5
    \override LyricExtender.after-line-breaking = #(conditional-kill-lyric-extender-callback)
  }
}

autoextenders =
#(define-music-function (lyrics) (ly:music?)
   (let* ((has-hyphen?
           (lambda (event)
             (let* ((art (ly:music-property event 'articulations))
                    (is-hyphen? (lambda (ev) (eq? (ly:music-property ev 'name) 'HyphenEvent))))
               (find is-hyphen? art))))
          (has-extender? 
           (lambda (event)
             (let* ((art (ly:music-property event 'articulations))
                    (is-hyphen? (lambda (ev) (eq? (ly:music-property ev 'name) 'ExtenderEvent))))
               (find is-hyphen? art))))
          (add-extender! 
           (lambda (event)
             (ly:music-set-property! event 'articulations
               (append (ly:music-property event 'articulations) (list (make-music 'ExtenderEvent))))
             event)))
     (music-map
      (lambda (event)
        (if (and (eq? (ly:music-property event 'name) 'LyricEvent)
                 ; do not add extenders below hyphens
                 (not (has-hyphen? event))
                 ; do not duplicate extenders
                 (not (has-extender? event))
                 ; do not add additional extenders after _ (internally equivalent to " ")
                 ; to avoid killed extenders
                 (not (equal? (ly:music-property event 'text) " "))
                 ; do not add extenders after "" to allow ending extenders
                 ; within _ _ _ _ skip sequences with _ _ "" _ _
                 (not (equal? (ly:music-property event 'text) ""))
                 ; ditto, with _ _ \markup{|null} _ _
                 (not (equal? (ly:music-property event 'text) (markup #:null)))
                 )
            (add-extender! event))
        event)
      lyrics)))