trait Node {
  abstract final String name
  Node parent = null

  List<Node> children = [];

  Node(name: name) {
    this.name = name
  }

  @Override
  void setChildren(List<Node> nodes) {
    nodes.each { -> node
      node.parent = this
    }
    this.children = nodes
  }

  @Override
  List<Node> getChildren() {
    this.children
  }

  void addChild(Node node) {
    node.parent = this
    this.children << node
  }

  @Override
  Node getParent() {
    this.parent?
  }

  @Override
  void setParent(Node node) {
    if (this.parent?)
      throw new Exception("Parent already set for node: ${node}")
    this.parent = node
  }

  Boolean isRoot() {
    !this.parent ? true : false
  }
}


class Tree {
  List<Node> nodes

  void walk() {}
}