arpeggioBracket abuse

[Dan Eble]

NR section 2.1.5:

> Divided voices
> 
> Using arpeggioBracket to make divisi more visible
> 
> The arpeggioBracket can be used to indicate the division of voices where 
> there are no stems to provide the information. This is often seen in choral 
> music. 

This deserves an enhancement ticket.  Specifying \arpeggio on a chord that is 
not intended to be performed as an arpeggio stinks.  Lilypond should 
automatically add brackets where the allocation of notes to voices is unclear, 
when enabled by a context property.
— 
Dan