Is it right how oneVoice reverts direction settings?

[Simon Albrecht]

Hello everybody,

suppose I want to globally override the direction of slurs, as far as 
neutral voices go. If I do this:

%%%%%%%%%%%%%%% \version "2.19.80" \layout {   \context {     \Voice     
\override Slur.direction = #UP   } } music = { c'( d' e' f') } {   
\music   \voiceTwo   \music   \oneVoice   \music } %%%%%%%%%%%%%%%%

then in bar 3 the slur will go below the notes, i.e. \oneVoice reverted 
the direction without knowing that an override was in effect.

I’d love to be able to redefine \voiceTwo & Co. such as to imply 
\temporary, but that’s not currently possible:

warning: Cannot make PropertySet revertible.

Does anybody have an idea for another solution?

Best, Simon