code: function createEl (d, container) {
      var link = container.appendChild(createLink(d))
      if (d.children.length) {
        var list = createList(d.isCollapsed)
        d.children.forEach(function (child) {
          createEl(child, list)
        })
        link.appendChild(list)
      }
    }
docs: Create link and list elements.
  @param {Object} d
  @param {HTMLElement} container
  @return {HTMLElement}