static L<Tree<S>> indentedStringsToForest(LS l) {
  if (empty(l)) ret emptyList();
  int baseIndent = indentWidth(first(l));
  new L<Tree<S>> forest;
  for i over l: {
    int j = i+1;
    while (j < l(l) && indentWidth(l.get(j)) > baseIndent) ++j;
    forest.add(new Tree<S>(l.get(i), indentedStringsToForest(subList(l, i+1, j))));
  }
  ret forest;
}