Warn on solitary \! ?

[Simon Albrecht]

Hello,

I was just wondering if we might want to emit a warning if there is a \! 
without a (de)crescendo before. It’s likely that the latter has only 
been forgotten.

\version "2.19.42"
\displayMusic {
  2 4\!
}

=>

(make-music
  'SequentialMusic
  'elements
  (list (make-music
          'NoteEvent
          'duration
          (ly:make-duration 1))
        (make-music
          'NoteEvent
          'articulations
          (list (make-music
                  'CrescendoEvent
                  'span-direction
                  1))
          'duration
          (ly:make-duration 2))))

Happy to hear your opinions,
Simon