static L<S> standAloneLines(L<S> lines) {
  new L<S> l;
  for i over lines: {
    if (nempty(get(lines, i))
      && emptyAfterTrim(get(lines, i-1))
      && emptyAfterTrim(get(lines, i+1)))
      l.add(get(lines, i));
  }
  ret l;
}