code: function RBTSentinel() {
    this.key = null;
    this.value = null;
    this.color = BLACK;
    this.parent = null;
    this.left = null;
    this.right = null;
  }
docs: RBT Sentinel Node
  @constructor
  @ignore
  @property {null} key
  @property {null} value
  @property {Number} [color=BLACK]
  @property {null} parent
  @property {null} left
  @property {null} right