fun! RstLabelCompletion(findstart, base)
  if a:findstart
    " locate the start of the word
    let line = getline('.')
    let start = col('.') - 1
    while start > 0 && line[start - 1] =~ '\a'
      let start -= 1
    endwhile
    return start
  else
    let line = line('.')
    let col = col('.')
    call cursor(1, 1)
    let res = []
    while search('\.\. _' . a:base, "Wc") > 0
      let m = substitute(getline('.'), '.*\.\. _\(.*\):', '\1', '')
      call add(res, m)
    endwhile
    call cursor(line, col)
    return res
  endif
endfun
setlocal completefunc=RstLabelCompletion