code: function row (root, prefix, isTail, out, printNode) {
    if (root) {
      out(`${ prefix }${ isTail ? '└── ' : '├── ' }${ printNode(root) }\n`);
      const indent = prefix + (isTail ? '    ' : '│   ');
      if (root.left)  row(root.left,  indent, false, out, printNode);
      if (root.right) row(root.right, indent, true,  out, printNode);
    }
  }
docs: Prints level of the tree
  @param  {Node}                        root
  @param  {String}                      prefix
  @param  {Boolean}                     isTail
  @param  {Function(in:string):void}    out
  @param  {Function(node:Node):String}  printNode