Request: Accepting isolated durations as ly:music?

[Peter Crighton]

% As discussed on the User mailing list, isolated durations are currently not
% accepted as expressions of type ly:music?, thus making it impossible to
% create functions expecting either music or isolated durations with an
% arbitrary number of post events.
% http://lists.gnu.org/archive/html/lilypond-user/2016-02/msg00478.html

\version "2.19.33"

tieFixSlash = #(define-music-function (parser location rhythm tie)
                 (ly:duration? ly:event?)
                 #{
                   \shape #'((0.5 . 0) (0.5 . 0) (0.8 . 0) (0.8 . 0)) Tie
                   $rhythm $tie
                   \once \override TieColumn.tie-configuration = #'((2 . 1))
                 #})

\new Voice \with {
  \consists "Pitch_squash_engraver"
} {
  \improvisationOn
  \tieFixSlash 4~ 4 \tieFixSlash 4(~ 4)
}