A glissando which extends into several \alternative
blocks
can be simulated by adding a hidden grace note with a glissando
at the start of each \alternative
block. The grace note
should be at the same pitch as the note which starts the initial
glissando. This is implemented here with a music function which
takes the pitch of the grace note as its argument.
Note that in polyphonic music the grace note must be matched with corresponding grace notes in all other voices.