Ambitus with multiple voices
Adding the Ambitus_engraver to the Staff context creates
a single ambitus per staff, even in the case of staves with multiple
voices.
\new Staff \with { \consists "Ambitus_engraver" } << \new Voice \relative c'' { \voiceOne c4 a d e f1 } \new Voice \relative c' { \voiceTwo es4 f g as b1 } >>