Problem with rest collision

[Jefferson dos Santos Felix]

I try to remove the rest collision of the example below, but no success:

\version "2.8.1"
\score
{
 <<
  {a8 d' f' a'} \\ {r8 c' d' r}
 >>
}

I try to use this:
 \override Score.RestCollision #'minimum-distance = #1.75

but the collision persists.

How can I solve it?

Thanks,

-- 
Jefferson dos Santos Felix
-----------------
felixmusic.net