how to force extra accidental in this case

[Stefan Thomas]

Dear community,

how can I force an extra accidental in the upper voice in the below quoted context?

I would like to do it in the defintion of the variable called accidentals.

\version "2.18.2"

Uppernotes = { \repeat "unfold" 8 { cis'8 } }

accidentals = { s2 s!4 s } % doesn't work

LowerMusic = { r2 c'!8 r r4 }

UpperMusic = { \context Voice << { \Uppernotes } {\accidentals } >> }

\score {

\new PianoStaff

<<

\new Staff \UpperMusic

\new Staff \LowerMusic

>>

}