code: function SHAMapTreeNodeLeaf(tag, data, type) {
    SHAMapTreeNode.call(this);
  
    if (typeof tag !== 'string') {
      throw new Error('Tag is unexpected type.');
    }
  
    this.tag = tag;
    this.type = type;
    this.data = data;
  }
docs: Leaf node in a SHAMap tree.
  @param {String} tag (equates to a ledger entry `index`)
  @param {String} data (hex of account state, transaction etc)
  @param {Number} type (one of TYPE_ACCOUNT_STATE, TYPE_TRANSACTION_MD etc)
  @class