bug or doc: Completion_heads_engraver doesn't do rests

[Graham Percival]

Completion_heads_engraver doesn't work on rests.  Is this a bug
or an undocumented feature?  :)


\version "2.5.17"
\new Voice \with { \remove "Note_heads_engraver" \consists 
"Completion_heads_engraver" }
{ r2. r2 r2. }