Omitted rests should not collide with anything

[Pavel Roskin]

If rests are omitted, they should not collide with anything.  But they  
do.  Example:

\version "2.17.20"
\score {
  <<
    \new Staff
    <<
      \new Voice { r4 }
      \new Voice { \omit Rest r4 }
    >>
  >>
}

I get a warning:

rests.ly:6:20: warning: cannot resolve rest collision: rest direction not set
      \new Voice {
                   r4 }

--
Regards,
Pavel Roskin