static S renderConcept(Concept.Ref ref) {
  ret renderConcept(ref.get());
}

static S renderConcept(Concept c) {
  if (c == null) ret "null";
  new StringBuilder buf;
  buf.append(c.id + " " + shortDynamicClassName(c));
  for (S field : conceptFields(c)) {
    O val = cget(c, field);
    if (val != null) {
      if (eq(val, Bool.FALSE) && isPrimitiveBoolField(c, field)) continue;
      buf.append(" " + field + "=");
      buf.append(renderConcept_short(c, val));
    }
  }
  ret str(buf);
}

// renders a value in a concept
sS renderConcept_short(Concept c, O val) {
  if (val instanceof Concept)
    ret shortDynamicClassName(val) + ((Concept) val).id;
  else if (val instanceof Concept.RefL)
    ret squareBracket(joinWithComma(lmap(v -> renderConcept_short(c, v), (L) val)));
  else {
    new structure_Data data;
    data.seen.put(c, 0); // for ConceptInList et al
    ret struct(val, data);
  }
}