fold-expr not working as expected

Question

I have trouble with one specific folding expression, s1. The documentation (:h fold-expr) says:

"s1", "s2", .. subtract one, two, .. from the fold level of the previous line

However this is not true for me. Create a file .vim/after/ftplugin/vim/test.vim/ with the following content:

"" This is a demo, the fold starts here. This line has foldlevel 1
" This line is folded wrong, it should have foldlevel 0, but has foldlevel 1
function! VimFolds(lnum)
  let thisline = getline(a:lnum)
  if match(thisline, '^""') >= 0
    return '>1'
  elseif match(thisline, '^" ') >= 0
    return 's1'
  else
    return '='
  endif
endfunction
setlocal foldmethod=expr
setlocal foldexpr=VimFolds(v:lnum)
set foldcolumn=3
set foldminlines=0

Open the file with Vim, it should be folded according to its own rules. The second line should be on foldlevel 0 according to documentation, but is on foldlevel 1.

Am I understanding something wrong here on how to use this?

Answer 1

"" This is a demo, the fold starts here. This line has foldlevel 1
" This line is folded wrong, it should have foldlevel 0, but has foldlevel 1
function! VimFolds(lnum)
  let thisline = getline(a:lnum)
  if thisline =~ '^""'
    return '>1'
  elseif thisline =~ '^" '
    return foldlevel(a:lnum-1)-1
  else
    return foldlevel(a:lnum-1)
  endif
endfunction
set foldmethod=expr
set foldexpr=VimFolds(v:lnum)
set foldcolumn=3
set foldminlines=0

I don't know how to make the 's1' value work as intended either.
However the code above seems to do what you want.