docs: public void checkParentLinks() {
          this.visit(new NodeVisitor() {
              @Override
              public boolean visit(AstNode node) {
                  int type = node.getType();
                  if (type == Token.SCRIPT)
                      return true;
                  if (node.getParent() == null)
                      throw new IllegalStateException
                              ("No parent for node: " + node
                               + "\n" + node.toSource(0));
                  return true;
              }
          });
      }