/* input e.g. [[
prelude

title
-----

text

title 2
-------

text
]] */

static SS underlinedHeadingSections(S text) {
  LS lines = lines(text);
  L<Int> l = indicesOfStartingWithTrim(lines, "--");
  new LinkedHashMap<S> out;
  if (nempty(l))
    mapPutIfNemptyValue(out, "", rtrim(lines(takeFirst(lines, first(l)-1))));
  for i over l: {
    int i1 = get(l, i);
    int i2 = or(get(l, i+1), l(lines)+1);
    S heading = trim(get(lines, i1-1));
    out.put(heading, lines(subList(lines, i1+1, i2-1)));
  }
  ret out;
}