code: function Iterator(tree, snapshot) {
    this.tree = tree;
    this.root = snapshot;
    this.current = snapshot;
    this.key = null;
    this.value = null;
  }
docs: Iterator
  @constructor
  @ignore
  @param {RBT} tree
  @param {RBTNode} snapshot
  @property {RBT} tree
  @property {RBTNode} current
  @property {Object} key
  @property {Object} value