Slur position : "catching" Llilypond values...

[Éditions IN NOMINE]

Well, let me explain  :-) 

I often use \once \override Voice.Slur #'positions = #'(a . b) to adjust Slur 
position, and most of the time, I only need to modify only one of a or b.

I'd like to be able to "catch" the default a or b value and tell Lily not to touch it, so that I could tell Lily something like : 

\once \override Voice.Slur #'positions = #'(* . b)

with * = default value, which would mean "whatever you want"

Does anybody know a clever trick for that ?

Best regards !
JMarc