Weird tie behaviour when changing Tie_engraver context

[José Luis Cruz]

I was very happy because of finding a way to make ties between voices,
simply changing the context of Tie_engraver from Voice to Staff. But
I've found a non desired consecuence of that.

Look at the example I provide. There are 2 voices. If I tie the first
one, ties are created also in the second voice. And viceversa, if I
tie the first chord of second voice, a tie is created for the c''1 on
first voice... For me that's a bug.

\version "2.11.20"
\score {
<<
 {c''1~ | c''4 d'' e''2}
 \\
 {<a d' f'>1 | <a d' f'>1}
> >
 \layout {
       \context {\Staff \consists "Tie_engraver"}
       \context {\Voice \remove "Tie_engraver"}
     }
}

cheers,

Jose