children(v = GRAPH_NODE) {
    if (this._isCompound) {
      var children = this._children[v];
      if (children) {
        return Object.keys(children);
      }
    } else if (v === GRAPH_NODE) {
      return this.nodes();
    } else if (this.hasNode(v)) {
      return [];
    }
  }
Gets list of direct children of node v.
Complexity: O(1).