sclass Rule {
  new L<S> in;
  new L<S> out;
  
  *() {}
  *(L<S> *in, L<S> *out) {}
}

static L<Rule> parseRules(S text) {
  L<S> log = toLinesTrim2(text);
  L<Int> emptyLines = concatLists(ll(-1), indexesOf(log, ""), ll(l(log)));
  new L<Rule> rules;
  for (int ii = 0; ii+1 < l(emptyLines); ii++) {
    int i = emptyLines.get(ii), j = emptyLines.get(ii+1);
    L<S> l = subList(log, i+1, j);
    continue if empty(l);
    L<Int> arrows = indexesOfLinesStartingWith(l, "->");
    if (empty(arrows)) continue;
    
    // it's a rule
    for (int ia : arrows)
      l.set(ia, dropPrefixTrim("->", l.get(ia)));
    rules.add(new Rule(
      subList(l, 0, first(arrows)),
      subList(l, first(arrows))));
  }
  ret rules;
}