\break not working when grouped

[David Bobroff]

Just noticed that the following construction no longer works.  Shouldn't
I see a single measure on each line with this?

-David

\version "2.5.18"

mus = \relative c' {
  c d e f g a b c
}

\score {
  \context Staff <<
    \mus
    s1 \break
  >>
}