\accidentalStyle for common choir notation

[Jonathan Scholbach]

Hi Ponders!

I would like to know how I can create my own \accidentalStyle
"Normally" notes of a certain voice are "naturalized" (written with a natural), when the same note had occured with an accident in the same bar (this is \accidentalStyle default in LilyPond). A different style additonally naturalizes notes, when the same note had occured with an accident in the antecedent bar of this certain voice. (this is \accidentalStyle modern in LilyPond)

I would like to create an \accidentalStyle which is very common in setting choir-music:
It shall naturalize accidentals that had taken place in the antecedent bar of any Voice in the same \StaffGroup (or \ChoirStaff, wich makes no big difference, I guess),

A tiny example demonstrating what I want is written below. In bar No. 2 the c of the Soprano is naturalized - due to \accidentalStyle normal. I want to behave the Alto voice in bar No. 4 analogously - i.e. as printed in the example, but without the need to write c! in the input of the alto voice.

Can someone please help me? I grubbed myself through the manuals and was still unable to find a way to write my own \accidentalStyle

Thank you!

Jonathan

\version "2.19.22"

soprano = \relative c'' {
 cis1
 c
 cis
 e
}

alto = \relative c'' {
  e1
  e
  e
  c!
}

\score {
  \new StaffGroup <<
    \new Staff <<
      \accidentalStyle modern
      \set Staff.instrumentName = "Soprano"
      \new Voice = "upper" { \soprano }
    >>
    \new Staff <<
      \accidentalStyle modern
      \set Staff.instrumentName = "Alto"
      \new Voice = "lower" { \alto }
    >>
  >>
}