// multi-level structureLines
static S structureTree(O data) {
  if (data instanceof O[]) data = asList((O[]) data);
  
  if (data instanceof Collection) {
    new L<S> l;
    l.add(shortClassName(data));
    for (O x : (Collection) data)
      l.add(structureTree(x));
    ret indentx(fromLines(l));
  }
  
  ifclass MultiSet
  if (data instanceof MultiSet) data = multiSetToMap(data/MultiSet);
  endif
  
  if (data instanceof Map) {
    new L<S> l;
    for (O key : ((Map) data).keySet()) {
      O value = ((Map) data).get(key);
      l.add(structureForUser(key) + " = " + structureForUser(value));
    }
    ret fromLines(l);
  }
  
  ret structureForUser(data);
}